What is an ADU?

In New York City, an ADU is a self-contained living unit that's built on the same zoning lot as another one- or two- family home. Think of an ADU as an extra home, like a backyard cottage, a basement, or apartment over a garage.

It has its own kitchen, bathroom, and entrance. An ADU is compact, measuring no more than 800 square feet and intended for residential use only.

What kinds of ADUs can you build in NYC?

New York City allows homeowners to create one ADU per single-family or two-family house. Within that framework, you can choose from several types of ADUs, depending on your property and goals. The main options include detached units in the backyard, attached additions to the main building, and interior conversions such as attic or basement apartments. Each type has its own design possibilities in concert with zoning and building code requirements, which are explained in the sections below.

Detached

A detached ADU is a separate building in the rear yard. This could be a newly built ADU, a conversion of an existing garage to an ADU, or the creation of an ADU over a garage. Detached ADUs offer great privacy and flexibility.

Detached Conversion

An existing structure, like a garage or shed, that's converted or rebuilt into a new living space. Under zoning rules, you can often demolish and reconstruct it while still treating it as a conversion.

Detached New Construction

A completely new structure built in your backyard that adds living space where none existed before.

Attached

An attached ADU is an addition that abuts to and expands the footprint of your house. It might run beside the main building in the side yard or extend into the rear yard. This is a separate home with its own entrance that either shares a wall with the main building or sits directly beside it with its own fire-rated wall.

Attached Conversion

An existing part of your home, such as a porch, garage, or side addition, that is enclosed or rebuilt to become a separate living unit. It reuses what already exists to create a legal ADU.

Attached New Construction

A new addition built onto your home that creates a separate living space connected to the main structure.

Attic

An attic ADU is an apartment built within your attic that has enough headroom. It uses space under the roof without changing the footprint of your property. Attic ADUs specifically require automatic sprinklers.

Basement
and Cellar

A basement or cellar ADU is an apartment below the first floor. To qualify as a legal ADU, the basement and cellars must meet safety requirements for ceiling height, light and ventilation, separate access, and fire protection. Basements and cellars have their own different requirements for conversion.

New Basement and Cellar

A new basement or cellar ADU is built below an existing structure and must meet strict structural, waterproofing, and safety codes. Excavation and foundation work can make this one of the most complex and costly ADU options.

Basement and Cellar Conversions

A basement that is finished or remodeled to create a new, separate living unit within an existing home. The space may already have utilities or partial finishes but requires upgrades to meet ADU safety and code standards.

Existing Basement and Cellar Legalization

An existing basement or cellar apartment that was previously used as housing without permits and is being brought into compliance with current zoning and building codes to become a legal ADU.

What is a Rear Yard ADU?

A rear yard ADU is a freestanding home (detached ADU) that sits behind your main building or a home attached to your main building (attached ADU).

Only some homes in NYC can have a rear yard ADU. The 'ADU for You' Eligibility Lookup Tool can help determine if you're eligible to build a rear yard ADU.

Why build
an ADU?

An ADU is more than extra square feet. It's a way to adapt your home to changing needs, expand opportunities, and invest in the future.

Generate income.

Create an additional renal unit on your property to possibly bring in additional income.

Live across generations. Bring family closer.

An ADU allows aging parents or adult children to live “next door” while still enjoying a sense of privacy.

Age in place. Stay rooted in your community.

A smaller, accessible home on your property lets you downsize while staying in your neighborhood. Explore more about Aging in Place.

Add space and value.

An ADU can help make better use of your zoning lot and create flexible space that can adapt to your needs over time. It can also add to your property's long-term value.

Support gentle growth for your neighborhood.

Adding an ADU means adding affordable housing. You provide more choices for people, strengthening both your block and the city.

Room Labels

Dimension String

Understanding key dimensions can help you understand the size of rooms and other design elements.

Doors

The direction of the arc represents the direction of the door swing.

Wall Thickness

Thick lines typically represent exterior or structural walls, while thinner/skinnier lines represent interior partitions.

Windows

Windows in plan are represented by breaks in the wall. Windows are typically represented with a series of parallel lines.

Furniture and Fixtures

Furniture and fixtures like toilets and sinks show fixture location but also demonstrate the scale of what can fit in each space.

What is the process for building your own ADU?

Your ADU journey starts here. This is your chance to imagine how your home can grow with you. Every step is part of a conversation with your family, your architect or engineer, or the city agencies guiding you toward creating a legal, livable home. The following outlines your ADU building process, from determining eligibility to occupying it.

Check Your Eligibility

Look up your address

Before diving into design ideas, check if your property is eligible to add an ADU. In NYC, your eligibility depends on a few factors. Is your home a one- or two-family residence? Does it fall within a flood zone? Check the ADU for You Eligibility Lookup Tool to find out what is allowed on your lot. Also, visit our rules and regulations page for more information.

Understand your available square footage

Once you know you’re eligible to build an ADU, the next step is figuring out how much space you can build and check whether you have already maxed out your buildable zoning floor area. For attic or basement units, the size usually fits within the area of your existing floor. For rear yard ADUs, zoning rules decide the size of the footprint. Use the ADU for You Eligibility Lookup Tool to estimate your buildable area. You can also check our rules and regulations to calculate it yourself. Don’t forget to verify the dimensions of your rear yard or home.

Determine
Your Goals

Understand how an ADU can benefit you

Start by thinking about what you want from an ADU. Do you need space for a relative, a rental apartment for extra income, or a long-term investment in your property? Understanding your goals helps you consider your options. This way, you can make informed choices about size, layout, and budget.

Understand your available square footage

Building an ADU from design to completion of construction can take anywhere from two months to two and a half years. The timeline depends on factors specific to your project, including design complexity, property conditions, and permitting, as well as how quickly you make decisions and stay involved throughout the process. It’s best to plan for a flexible timeline rather than a firm deadline since unexpected challenges can arise in any site-specific construction project.

You’ll need to hire a Registered Design Professional (RDP), a construction crew, and specialists like plumbers and electricians during this time. You’ll also connect with city agencies like the Department of Buildings (DOB), Department of Environmental Protection (DEP), and Department of Housing Preservation and Development (HPD). As the project unfolds, you’ll need to monitor progress, coordinate with your design professionals, and consider the best ways to finance the work.

Plan Your ADU

Survey your property

Before moving forward with ADU plans, hire a professional NYS-licensed land surveyor or building surveyor to confirm your lot and house size and assess your home’s existing conditions. It is important to get accurate information about your utilities, easements, contours, and spot elevations at a minimum. For interior ADUs, it’s important to understand your home’s existing structure and overall condition. Assess the strength of the framing, the location of load-bearing walls, and the condition of floors, ceilings, and foundations before beginning design
or construction.

Choose a pre-approved or custom design

Depending on the type of ADU you plan to build, you may have access to several methods to build your ADU. You could use a plan from the City’s Pre-Approved Plan Library (PAPL), or you can choose to work with a Registered Design Professional to create a custom design. Another option is to buy a manufactured home and have it installed in your rear yard.

Understand
Your Budget

Your budget will shape when and how you move forward with an ADU. Be sure to account for both short- and long-term costs, from construction expenses to potential property tax and maintenance increases. Some income-eligible homeowners may qualify for city and state-funded programs, such as the Plus One ADU Program, which may provide financial assistance and support with feasibility planning and project management. For a deeper look, refer to Chapter 4 on ADU costs and use the pro forma tool to estimate costs.

Build Your ADU

Screen for a Registered Design Professional

Whether you opt for a PAPL design, a manufactured home, or a custom plan, it’s essential to find an RDP who works well with you and suits your project. This guide includes questions and tips to help you before you sign a contract to set clear expectations and confirm their services align with your ADU goals.

Contract with a Registered Design Professional

Once you confirm your property is eligible, set your goals, pick a design, and plan your budget. Then hire an RDP.

Get more information and guidance on selecting an RDP on the DOB website.

Approve and permit your design

After your ADU design is complete, your RDP will submit an application to the Department of Buildings (DOB) for approval. Once DOB approves your plans, construction permits can be issued. If you’re using a design from the PAPL, this application process may possibly be faster and less costly since the plans have already been reviewed by DOB. If you’re not using a PAPL design, your RDP will prepare and submit all necessary documents for a full DOB review before permits are issued. For more details, see What Is the ADU Permitting Process in NYC?

Construct your ADU

It’s time to start construction. Use the construction timeline to identify key milestones, and phases and review the quality assurance cheat sheet to understand what to look out for throughout the process.

Prepare to
Move-In

Conduct inspections

Once construction is complete, your contractor will schedule a visit with a NYC building inspector to confirm your ADU is safe and ready for occupancy. For more details, see our section on what to expect during an inspection.

Learn best practices for being a landlord

If you build your ADU to rent, remember that being a landlord means you have important on-going responsibilities. Check the landlord responsibilities section for an overview and links to city resources. These resources can help you and your tenants.

ADU Eligibility
Lookup Tool

Use the ADU for You Eligibility Lookup Tool to see if it is possible to build an ADU on your property. Enter your address to see which ADU types suit your lot. Also, check out the Pre-Approved Plan Library for designs that fit within your property’s allowed boundaries.

Can I build
an ADU?

Your eligibility is based on site-specific rules and laws. This includes the New York City Zoning Resolution and Building Code. This chapter outlines the main rules that determine if your property qualifies, the types of ADUs allowed, and the design limits. If you want to dig deeper, you’ll also find a short guide to zoning and building requirements beyond your lot, plus links to the city’s zoning regulations, building construction codes, and local laws.

Where are ADUs allowed in NYC?

The map below identifies areas of New York City where Rear Yard ADUs are and are not currently allowed. The color-coded areas highlight historic districts, flood risk zones, and neighborhoods outside the city’s Greater Transit Zone.

Flood Zone

Basement, cellar, and Rear Yard ADUs are not allowed in flood risk areas identified by the Department of Environmental Protections. Click here to see if your property is in a flood risk area on the NYC Interim Flood Risk Mapper.

If you believe your proposed ADU meets flood safety standards, you may request a determination from the DOB. If DOB denies your application based on flood risk, you can appeal to the NYC Board of Standards and Appeals (BSA) under Charter Section 666 (6) (7). Visit the Board’s website for the NYC Board of Standards and Appeals (BSA).

Other Restrictions

Rear Yard ADUs and zoning limits

Rear Yard ADUs are not allowed in zoning districts R1-2A, R2A, and R3A outside the NYC Greater Transit Zone. They are also not permitted in Historic Districts designated by the Landmarks Preservation Commission (LPC), unless you’re converting an existing structure, such as a garage, into an ADU. In those cases, additional review and approvals from LPC will still be required

Zoning regulations and guidelines

Before you design or build an ADU or contract to purchase and install a manufactured ADU, you need to know the zoning rules that apply to your property. These regulations determine where ADUs are allowed in New York City, how big they can be, and where they can be sited on your property. They also set standards for light, air, and neighborhood context through rules on height, yard space, and building separation.

This section translates key zoning definitions from Chapter 2 and Chapter 3, Sections 23-341 and 23-371 of the NYC Zoning Resolution into plain language you can apply to your own lot.

Lot line offsets

ADUs must be located at least 5 feet from all rear and side lot lines.

Zoning Resolution — Chapter 3, 23-341 and 23-371

Distance between new and existing structures

There must be at least 10 feet of separation between the primary house structure and a detached ADU. The distance helps maintain light, air, and safety between buildings. While larger homes, including those over 35 feet tall, may require greater separation from other residential buildings, the 10-foot minimum applies specifically to ADUs and their primary homes.

Zoning Resolution — Chapter 3, 23-341 and 23-371

How big can
my ADU be?

Your ADU cannot exceed 800 square feet of total floor area.

For ADUs that are being added to a detached, semi-detached or zero lot line building the height is capped at one story. It can’t go over 15 feet, measured to the highest point of the roof.

If the ADU has a parking space or garage below, it can be two stories tall. However, it must not go over 25 feet, measured at the highest point of the roofline.

A detached ADU cannot take up more than one-third of your property’s rear yard. The rear yard is the open space at the back of your property measured 20 feet from your rear lot line. For example, to build a 500-square-foot ADU, your rear yard must be at least 1,500 square feet.

What other
key zoning regulations do I need to know?

Your zoning lot must be your primary residence upon the initial occupancy of your built ADU.

Detached ADUs must have direct access through a side yard at least five feet wide.

The PAPL may help you reduce some costs

One of the biggest advantages of New York City's Pre-Approved Plan Library (PAPL) is possible cost savings. Because the plans are already reviewed for general zoning and code compliance, the PAPL may help reduce soft costs like extended reviews, repeated filings, and added consultant time. This means projects may move faster and budgets may be more predictable. To see how this works in practice, explore our Pro forma tool to explore different scenarios.

Soft Costs

Surveyor

Most projects start with a property survey. This checks lot boundaries, dimensions, and site conditions and helps ensure your ADU design will meet zoning rules. It also helps you avoid issues with neighbors. A site topographic and utility survey will be required for a DOB application.

Architectural fees
(May be reduced for Pre-Approved plans)

If you hire an architect or engineer for a custom design, they will work with you to design an ADU that fits the site of your home. Choosing a pre-approved plan from the Pre-Approved Plan Library can make that process faster and simpler since some of the design and compliance reviews are already complete.

Engineering fees
(May be reduced for Pre-Aproved plans)

Structural, mechanical, or civil engineers may need to sign off on the design. These services help ensure the ADU is safe and up to code. Pre-Approved plans may reduce the need for additional engineering review.

Insurance for the building process

Construction insurance, also called builder's risk insurance, covers damage or accidents during construction and protects both you and your contractor. In most cases, the contractor carries this policy, but some lenders or insurers may require the homeowner to hold it instead. Before construction begins, confirm with your Registered Design Professional, contractor, and insurance provider who is responsible for securing the policy.

Submetering

Some utilities, particularly water and electricity, may require service upgrades or panel modifications to accommodate separate metering. Coordinate early with utility providers to confirm feasibility and compliance.

Approval and perming fee

Every ADU requires permits from the Department of Buildings. Using a pre-approved plan can speed up review. Depending on your site, you may also need additional permits from other city agencies, such as the Department of Environmental Protection (DEP) or the Fire Department. The cost of the insurance policies a General Contractor (GC) is required to have to "pull a permit" may be a cost the GC wants the owner to pay. Regardless, the cost for the GC insurance will be part of the construction cost.

Property insurance

Once the ADU is complete, you'll need to adjust your homeowner's policy to cover the added structure.

Expected tax increase

Because the ADU raises your property's assessed value, you should plan for higher annual property taxes.

Potential
Soft Costs

Testing consultant

Some properties may require professional testing to ensure safety before construction. This can include soil analysis, structural assessments, or environmental testing for hazards like lead, asbestos, or vapor intrusion. If your ADU will be partly or fully below ground, you must conduct radon and vapor testing to meet city health and building standards.

Temporary utilities

Contractors may need temporary water or electricity during construction.

Hard Costs

Building materials

Materials include lumber, roofing, windows, insulation, and finishes. The cost of materials varies depending on quality and market conditions.

General contractor

Your general contractor manages the construction of your ADU from start to finish. They coordinate subcontractors, order materials, and ensure the project meets code and safety standards. Selecting a licensed and insured contractor is essential. Ask for references, check their permit history, and confirm that they are familiar with ADU construction in New York City. The general contractor's fee is typically a significant part of your overall budget and should include both labor and project management costs.

Equipment rental

Some ADU projects may require specialized equipment for excavation, lifting materials, or the setting of prefabricated units in place. These costs are often included in your contractor's estimate but can vary depending on the type of ADU and site conditions.

Foundation excavation

Depending on the type of ADU you are building, excavation may be required for the foundation of your ADU.

Utility hookups

Connecting your ADU to city services such as water, sewer, and electricity requires licensed professionals and city approvals. These connections link your new unit to the main lines and should be factored into your hard costs, while permit and inspection fees count as soft costs. Actual expenses depend on the distance to existing service lines and the condition of current connections.

Mechanical /electrical/plumbing (MEP) labor

Once utilities reach your ADU, licensed tradespeople install and connect the interior systems that make it livable. This includes wiring for lighting and outlets, plumbing for kitchens and bathrooms, and heating and cooling systems that tie into or operate independently from your main home.

Restoration work

Construction can affect existing yards, driveways, and sidewalks. Once the main work is complete, you may need to restore landscaping, repair pavement, or regrade soil for proper drainage. A new rear yard ADU will trigger builders pavement plan, front yard planting and street tree requirement per zoning.

Demolition

If you're converting or rebuilding an existing structure, such as a garage or shed, partial or full demolition may be required before construction can begin. In New York City, demolition is regulated by the Department of Buildings (DOB), the Department of Environmental Protection (DEP), and the Fire Department (FDNY). Each agency may require permits or inspections to address air quality, asbestos removal, and fire safety. Always confirm which approvals apply to your project before starting any demolition work.

Site work

All exterior ADUs require site preparation before construction. This may include grading soil for drainage, pouring new foundations or footings, and installing walkways or retaining walls. For basement or attic ADUs, site work might involve improving exterior access routes or adding new stairs. Proper site work ensures the new ADU integrates safely and smoothly with your existing property.

Sprinklers

ADUs in New York City are required to have a sprinkler system. There are specific requirements for basement and cellar apartments units. Installation must be done by professionals and approved through DOB inspections.

Attic work

If you're planning on converting your attic into an ADU, it's important to keep in mind that your roof may require replacement or repair. Bringing an attic ADU up to code also requires proper insulation that meets New York City's energy standards. Explore information related to insulation standards.

How Can
I Finance
my ADU?

Adding an ADU is a major investment, but homeowners have multiple ways to cover costs.

Conventional financing options include:

Home equity loans

This type of loan lets you borrow money based on how much of your home you already own. You receive the money to design and build your ADU all at once and pay it back in fixed monthly payments, like a second mortgage.

Home equity lines of credit (HELOC)

A HELOC gives you access to a flexible line of credit that you can draw from as needed during your project. It uses the value of your home as security. You can take out money as you need it during your project and pay interest only on what you use.

Construction loans

A construction loan provides short-term financing to cover building costs while your ADU is under construction. Once the project is complete, it often converts into a standard mortgage or long-term loan.

Cash-out refinancing

With a cash-out refinance, you replace your current mortgage with a new one that's bigger. You get the difference in cash, which you can use to pay for your ADU, but you'll start a new mortgage in the process.

Personal loans

A personal loan gives you money upfront without using your home as collateral. It may be faster to get but may come with higher interest rates and shorter repayment terms.

These are widely available through banks and credit unions and can be paired with City and State programs designed specifically for ADU development.

The Plus One ADU Program

The City's flagship financing tool is the Plus One ADU Program, which combines capital loans and grants to help eligible homeowners build or legalize ADUs.

Key features include:

• Up to $220,000 in loans and $175,000 in grants available.
• Loan terms of up to 15 years, with potential extensions to 30 years.
• Interest rates as low as 0% for eligible borrowers.
• Preference for owner-occupants earning up to 120% of Area Median Income (AMI), with eligibility extending to 165% AMI.
• Forgivable loan options tied to rent restrictions and capped annual rent increases, depending on load structure.
• Monthly payment safeguards ensure homeowners retain at least $200 in monthly cash flow after debts.

To qualify, homeowners must occupy their home, be current on mortgage payments, and have no outstanding municipal arrears.

City Incentives

New York City offers a variety of programs to support you when your ADU project contributes to sustainability, housing growth, or first-time rental opportunities. These programs provide financial incentives, tax benefits, and technical assistance that can help offset costs and ease the process of becoming a landlord.

Sustainable building incentives

NYC Accelerator

Free expert guidance, training, and resources to help building owners improve energy efficiency and comply with local energy laws. Connects homeowners with rebates and financing opportunities.

Densification
and affordable housing

NYSERDA incentives

Programs supporting multifamily retrofits, renewable energy, solar, and electrification projects, with options for affordable housing and smaller properties.

First-time and small landlords

HPD & HCR ADU Programs

Grants and financial support for homeowners creating ADUs, especially valuable for those becoming landlords for the first time.

The Homeowner Help Desk (HHD)

The Homeowner Help Desk (HHD) provides free support to low- and middle-income homeowners across New York City. Scammers often target households facing financial strain, but HHD connects you with trusted housing counselors and legal experts who can help you protect your home and your future. They may also help guide you through your ADU costs and loan process.

The Homeowner Help Desk may assist you with:

• Preventing foreclosure and managing mortgage payments
• Protecting against scams, deed theft, and predatory lending
• Resolving property tax and water bill debt
• Applying for low-cost loans and home repair resources
• Improving energy efficiency and accessing upgrade programs
• Creating an estate plan to keep your home in the family
• Understanding flood risks and insurance options
• Managing your home as a small landlord

For Neighbors
and Family

You don't need to be a homeowner yourself to make a difference. HHD offers workshops and training to help you recognize the signs of scams and deed theft so you can support friends, family, and neighbors.

Renting out
your ADU

You may have built your ADU with the goal of renting it out. In New York City, rental rules depend on whether you plan on offering the apartment for a short-term stay (fewer than 30 days) or a long-term lease (30 days or more). Understanding the difference is key to staying compliant and protecting yourself as a landlord.

Short-term rentals
(fewer than
30 days)

Key rules for homeowner-hosts of short-term rentals:

Live in your ADU as your primary residence for at least 183 days per year if you plan to rent it as a short-term rental. You cannot use a separate ADU for short-term rentals while living in another unit on the same property.

• Be physically present during each guest's stay.
• Host no more than two guests at a time.
• Allow guests full access to the entire unit without locked internal doors.
• Avoid entire home rentals for fewer than 30 days in single-family homes, two-family homes, or apartments in larger buildings.
• Ensure your ADU meets all health, safety, and welfare requirements, such as space, fire prevention, access to natural light and ventilation under the NYC Building Code.

Penalties for illegal rentals:

• City fines range from $2,500 for first offenses to over $100,000 for repeat violations.
• New York State law adds fines starting at $1,000 for advertising illegal rentals in buildings with three or more units.
• Additional penalties under the City's Consumer Protection Law include fines of up to $3,500 per day and repayment of any earnings from illegal rentals.

Long-term rentals (30 days or more)

If you rent your ADU to a tenant for 30 days or longer, it falls under New York City and New York State landlord-tenant laws. These laws cover leases, rent regulations, eviction processes, and the basic rights and obligations of landlords and tenants. As a homeowner, you'll need to follow rules around habitability, security deposits, and notice periods.

What Type of ADU Is Best for Your Property?

The New York City Zoning Resolution determines which ADU types you can build and where, but your goals as the homeowner will guide which options make the most sense for you. Each type offers different advantages depending on how you plan to use the space and who will live there. The examples below can help you think through what fits your needs and can help you through the ADU design process.

Detached ADU

A detached ADU is a separate building in the rear yard. It can either be a new structure or a conversion of an existing garage or an outbuilding. Detached ADUs provide the greatest level of separation, privacy, and flexibility.

Consider a detached ADU if:

You want to create an independent rental unit that offers privacy for both you and your tenant. The separation between homes can make the unit more marketable and potentially increase its rental value.

You want to create an intergenerational household while maintaining a degree of separation and privacy.

You want to age in place. Detached ADUs can be designed to meet accessibility, and mobility needs and allow aging adults to downsize while staying in their neighborhood.

You want a straightforward permitting and design process by selecting an ADU from the PAPL.

You have a garage or rear-yard structure that can be rebuilt or adapted into a new living unit.

You want to build an ADU but would prefer to avoid triggering the Multiple Dwelling Law by adding an ADU to an existing 2-family home.

Rear Yard ADU (New Construction)

A rear yard ADU is a newly built home that sits behind the main building. It must meet zoning requirements for setbacks, height, and distance from the main house. his option gives you design flexibility and allows for a fully customized layout but will typically cost more than a conversion.

Detached conversion (Garage or Outbuilding)

A detached conversion repurposes an existing structure, like a garage, into a livable home. Conversions can be more affordable, but they often require upgrades for insulation, foundation, structure, and utilities. Some garages may need partial demolition or rebuilding to meet current codes.

Attached ADU

An addition that extends your house. It can run alongside the main building, extend into the rear yard, or connect with your existing home. This type of ADU acts as a separate home while sharing at least one wall with the main house. An attached ADU can also convert an existing interior space, such as a sunroom or enclosed porch, without increasing the building footprint. These units must include a fire-rated separation between the new living space and the main home to meet code requirements.

Consider an attached ADU if:

You are seeking to generate additional income through a long-term renter in a shared or connected building, depending on your home’s classification and fire separation requirements.

You are looking to foster an intergenerational household for either aging parents or an adult child that’s attached to the main building.

You want to maintain a shared yard or access while still offering a private entrance and living area.

Attic, basement
and cellar ADUs

An attic ADU makes use of space under the roof without changing the footprint of your property. A basement or cellar apartment is located below the main floor.

Consider an attic or basement ADU if:

You are seeking to generate additional income through a long-term renter by utilizing the existing structure of your home.

You are looking for a more cost-effective solution for building an ADU. Attic and basement ADUs are constructed within the existing structure of your home, which may result in savings over a new build.

You have limited space in your rear yard for an additional structure.

You are looking for more seamless integration into your existing home. Attic and basement ADUs are more discreet if you are interested in maintaining the existing character of your home in the neighborhood.

What ADU features are best for your goals?

Once you know which ADU type fits your property, vision, and goals, you can start thinking about design features that matter most to you. Common goals when creating an ADU include accessibility, parking, sustainability, and comfort. We recommend talking with your RDP or looking for these features in the PAPL design descriptions.

Accessibility and Universal Design

Check out the NYC Department from Aging’s Aging in Place Guide for Building Owners for more ideas.

• No-step/ground-level with wide entries
• Walk-in Showers
• Elevator or stair lift
• Assistive railings or handrails
• Wheelchair turning-radius

Preserve Parking

• Two story garage conversion
• Covered parking space integration
• Electric Vehicle (EV) charging

Sustainable Systems

• High-performance insulation and windows
• Solar panels for electricity production
• Green roof
• Rainwater harvesting
• All-electric appliances

Comfort and Livability

• Skylights, dormers, or roof windows
• Outdoor patio or porch
• Soundproofing

What is the PAPL? How Can You Use It?

The Pre-Approved Plan Library (PAPL) is an online resource where you can explore detached and attached ADU designs created by licensed professionals. Each design has been reviewed and approved by the NYC Department of Buildings (DOB), which may help speed up the early approval and permitting process for your specific site. When you choose a design from the PAPL, you must enter into a contract with the RDP who created it.

Working with a Manufactured ADU

Factory-manufactured housing, also known as prefabricated, follows different HUD code and regulations than homes built directly on-site in New York City. Many homeowners find this option appealing because it can significantly shorten the construction timeline and lower overall costs.

Manufactured homes certified by New York State or the U.S. Department of Housing and Urban Development (HUD) can be used as ADUs if they meet the requirements of BC Appendix U. Although these homes are regulated by the state and federal governments, the City reviews zoning, site placement, and connections such as utilities and foundations to ensure compliance with local standards. If you’re unsure whether a specific manufactured home qualifies, contact the company or RDP associated with the design to confirm compliance.

Where to
start with a
custom plan

You may need to consider designing a custom plan for your ADU for the following reasons:

• Interior ADUs, such as basement and attics, require custom plans due to the unique building features of each 1- and 2- family home.
• The Pre-Approved Plan Library does not include a design option that meets your vision or needs.

If your project requires a custom plan, start the process by researching and contacting a RDP. An RDP is a New York State licensed architect or engineer who will design your ADU, prepare the necessary drawings, and ensure your project meets all NYC Construction Code and Zoning Resolution requirements. The RDP is also responsible for signing and stamping your permit application drawings.

When selecting an RDP, you should check both disciplinary and licensure records to ensure you’re working with a qualified professional. You can search RDPs to verify licenses and to review their professional history by using the NYS Education Department’s Verification Search.

Before meeting with an RDP, it may be helpful to:

• Secure a recent property survey
• Evaluate your goals for building an ADU (for example: generating rental income, housing a family member)
• Determine a general budget range
• Refer to the questions to Selecting your Design Professional section for a list of questions to ask while interviewing your RDP.

The RDP will usually visit your site, manage the project, and create design drawings that reflect your goals. They also prepare the documents needed for filing with the Department of Buildings so your project can move forward with permits for construction.

An architectural plan is a drawing that shows a bird’s-eye view of a building or a space as if a horizontal slice were cut through it. For your ADU, it illustrates how rooms, walls, doors, and fixtures are arranged. Learning how to read a plan helps you communicate with your RDP and contractor so you can make confident, informed decisions about your design. The diagram below points out some key elements to look for in an architectural plan.

Selecting your Registered Design Professional

How to
choose well:

Verify Credentials

Use the Find Licensed Professionals tool to confirm your RDP’s license is current and check for disciplinary actions.

Ask for experience

Look for someone who has worked on ADUs, small residential additions, or residential conversions in NYC.

Review past work

Ask to see similar projects. You want to understand how they handle space, light, materials, and construction details.

Check references

Call previous clients; they can reveal how the RDP communicates and if they stay on schedule.

Meet before signing

Bring a property survey and talk openly about your goals, budget, and timeline.

Questions to
ask during
your meeting:

• Have you designed ADUs, residential rehabs, renovations, or home additions in NYC?
• How do you handle DOB filings and approvals?
• What services are included in your proposal?
• How do you estimate costs and manage revisions?
• What does your timeline from design to permit look like?
• How do you usually respond to delays in a project?
• What have you done previously when a project is delayed further than you expected?
• How often do you typically communicate with homeowners and about what aspects of the project?
• How much do you participate during the construction of a project?

Every ADU must have a checklist

All ADUs are required to have specific elements to make it a legally compliant and habitable space for living. These requirements include:

Kitchen

• The ADU must include a space for preparing meals including a stove, sink, and counter.
• It must meet all plumbing and ventilation requirements as set by the NYC Construction Codes

Bathroom

• A complete bathroom including a toilet, sink, and shower or bathtub must be present, with plumbing that connects to the sanitary sewer and an appropriate venting system.

Separate Designated Entry

• This entry must be a dedicated entrance separate from the entrance to any other existing units and cannot be more than 100 feet from your property street line.

Bedroom or Sleeping Space with Operable Windows

• A space designated for sleeping must have adequate natural light and ventilation. In basements and cellars, all rooms must have a window large enough for an escape during emergencies, as well as provide two means of egress.
• The window must meet minimum size, opening, and sill height requirements as defined in the NYC Building Code.

Connections to utility networks such as electricity and plumbing

• The ADU must be properly connected (and permitted) into the local utility networks: electrical, water supply, sewer, and where applicable, or HVAC systems.
• Every plumbing fixture (kitchen sink, bathroom sink, shower/tub, etc.) must have both hot and cold water as required by health, safety, and code standards.
• Water heaters, distribution, insulation, and pipe sizing must comply with code.
• All ADUs must include a sprinkler system. The requirements change between detached, attached, attic, basement, and cellar ADUs.

Understanding Multiple Dwelling Law (MDL) and costs

If you already have multiple units on your property or plan to add more with your ADU, your project may fall under New York State’s Multiple Dwelling Law. Two-family homes adding a fire-separated attached or detached ADU are exempt, but adding units beyond this can trigger extra building code requirements. These may include upgrades to fire separation, sprinklers, and egress, which can add to your project’s overall cost. Check with your RDP early to understand whether MDL applies to you.

Common misconceptions about ADUs

It’s important to know that not every extra structure or space on a property can qualify as a legal ADU. Sheds, treehouses, or unregulated basement and cellar apartments may seem similar but usually don’t meet the standards required for safe, habitable housing under NYC regulations. Informal basement and cellar apartments, for example, must be upgraded to meet all building and zoning requirements, including proper exits, natural light and ventilation, plumbing, and electrical systems, before they can be considered legal ADUs.

Environmental impacts of ADU

New York City needs more housing, but certain materials and designs can impact the building’s carbon footprint during construction and over the long term. As a homeowner, you can make choices that reduce these impacts. Thoughtful decisions about energy use, insulation, and building materials can make your ADU more sustainable and contribute to a healthier city.

The diagram below highlights some opportunities for reducing the carbon footprint of your ADU.

Help minimize energy loss

• Increase building energy performance. Ask your RDP about increasing your window and wall insulation performance above required standards.
• Use less windows! Windows allow for higher energy loss than walls. Talk to your architect about how to make your home light filled and airy without using floor to ceiling windows.
• Consider all-electric systems. Electric heat pumps and induction stoves can reduce your home’s carbon footprint and improve indoor air quality but may increase upfront electrical costs. Discuss whether your existing electrical panel and service can support full electrification.
• Choose efficient appliances and fixtures. Look for ENERGY STAR® rated appliances and WaterSense®-certified, low-flow fixtures to reduce both energy and water use over time.
• Locate your ADU. Talk to your RDP about which direction your property should face to get the best sun (and heat) in the winter and summer.

Manage
water on site

• Some ADUs may have an opportunity to collect stormwater or manage gray water. Talk to your RDP if you are interested in water management.

Choose less environmentally intensive building material

• Choose lower impact structures. Typically, structures made of wood have a lower negative environmental impact than buildings made from concrete or steel.
• Use local or regional materials. Ask your RDP to source materials that are within 500 miles of your home or closer.
• Lower impact materials. Depending on the materials of your building you can ask your RDP to work with low carbon concrete, wood from a sustainable forest, recycled steel, or steel from electric arc furnaces.

Choose low impact finishes

• Work with your RDP to choose finishes that are healthy for your body and the environment. Materials like vinyl are slow to decompose and require a large amount of processing energy. Ask about known health outcomes related to highly processed materials.

Save trees
when possible

• Whenever possible save existing trees on your property. If you must remove a tree, consider planting a new tree or contributing to a City initiative like the NYC Urban Forest Plan.

Environmental certifications

• Ask your RDP if the design is compatible with LEED v4.1 Residential: Single Family Certification, Passive House Design, Zero Energy Ready Home (ZERH), or ENERGY STAR standards.

Take advantage of opportunities to upgrade your existing home

• Consider using this investment on your property to improve the environmental impact of your existing home. Price out the options to replace your windows, improve the insulation in your roof, improve your utility efficiency, or install PV panels.

Permitting Plans

What is the ADU permitting process in NYC?

Every ADU project in New York City requires a building permit from the Department of Buildings (DOB) before construction begins. Your RDP, whether a licensed architect or engineer, will handle the filing process, coordinate reviews, and ensure your design complies with zoning and construction codes.

Understanding Permit Types

Most ADU projects use one of two permit categories filed through DOB NOW: Build.

• New Building (NB): Used when you are constructing a new, detached ADU on your property.
• ALT CO (formerly ALT1): Covers major changes that affect the building’s use, egress, or occupancy (for example, converting a two-family to a three-family house) and require a Certificate of Occupancy or a revision to it.

Your RDP will advise which category applies and they will prepare the required drawings. A DOB plan examiner will then review the submission for compliance. If there are objections, your RDP will respond with corrections until the plans are approved. Learn how to obtain a permit on the NYC DOB page. Explore how to use DOB NOW.

With a
Pre-Approved Plan

If you use a design from our Pre-Approved Plan Library, your RDP may be able to adapt to your site’s dimensions and submit it directly to DOB. These plans have already cleared key technical reviews, so approvals may move faster.

• Confirm eligibility with your RDP.
• Select the plan that fits your lot type and goals.
• File through DOB NOW: Build as a New Building or Alt-CO-GC job (GC means General Construction), including the plan number.
• Receive DOB application approval and permits.

Without a
Pre-Approved Plan

If your project requires a custom design, common for attic and basement ADUs, or even Rear yard ADUs, your RDP will prepare all drawings and technical documents from scratch.

• The RDP designs the ADU and confirms it meets zoning and construction code requirements, such as structure, and fire-safety design requirements.
• The plan is filed for DOB review and correction, if needed.
• After approval, your General Contractor can “pull” the building permit.

Next Steps

Once permits are issued, construction can begin. All ADUs must complete required inspections and obtain a Certificate of Occupancy or Letter of Completion before anyone can move in.

Attic, Basement, and Cellar ADUs

Attic ADUs must meet requirements for ceiling height, egress, fire separation, insulation, ventilation, and natural light. Your RDP files the project as an ALT-CO or New Building job depending on whether it is within an existing home.

Basement ADUs can be created in one- or two-family homes if they meet Building Code and Zoning requirements for safety, moisture control, flood risk, and adequate light and air. Your RDP can confirm whether your basement meets the conditions for a legal conversion.

Learn how to obtain a permit and explore the Project Guidelines for Owners. You can also connect with a NYC DOB Project Advocate for support with the process.

Basement Legalization Program
(Local Law 126 of 2024)

Local Law 126 establishes a 10-year compliance window for existing, previously occupied basement and cellar units in certain neighborhoods to become legal apartments. These legalization projects must meet the specific safety, flood, and habitability standards outlined in the law.

To see if your neighborhood qualifies for the legalization program, visit NYC’s Basement Conversion Pilot page and check your address.

You can also find more information related to legalization in Chapter 9: Legalizing Your Basement or Cellar.

Planning for construction while tenants are living in the basement or cellar

If renovation is happening while tenants are still residing in the building, you will need a Tenant Protection Plan (TPP). This is specific to basement and cellar legalization projects or legal 2-family unit additions. A TPP is a document that outlines what steps you and your contractor will take to protect tenants when construction or renovation happens while tenants are living in the building. Your contractor must submit a TPP to receive building permits. Learn more about what needs to be included in a TPP at Tenant Protection Plan - Buildings.

Asbestos requirements

If your home was built before 1987, you must complete an asbestos assessment before DOB will issue a permit. This ensures that no asbestos containing materials (ACM) will be disturbed during construction. If asbestos is to be removed, the NYC Department of Environmental Protect (DEP) will require permits for ACM removal. Explore the details on the DEP asbestos abatement page.

Legal Responsibilities

License and access agreements

In New York City, building close to another property often means you’ll need your neighbor’s cooperation. When your ADU construction requires access to an adjoining lot, whether to install scaffolding, protect a shared wall, party wall, or retaining wall, or underpin a foundation, you’ll need a written license agreement. These documents formalize permission, outline conditions, and protect everyone involved.

Because these agreements are legal documents, you should work with your Registered Design Professional (RDP) and an attorney to draft or review them. Your RDP can identify when an access agreement is required based on your site conditions and construction scope, and an attorney can ensure the terms protect both you and your neighbor.

What Access Agreements Cover

A clear and complete access agreement removes ambiguity. It plainly defines what you can do, how long you can do it, and how you’ll protect your neighbor’s property. Typical provisions include:

• Description of permitted work that explains what activities are allowed—such as excavation, façade protection, or repairs.
• Scope of access detailing which individuals or contractors may enter, what areas can be accessed, and what equipment can be used.
• Duration of license specifying start and end dates for property access, plus any extensions or termination clauses.
• Compensation for inconvenience, often ranging from a few hundred to several thousand dollars per month depending on risk and duration.
• Insurance and indemnification protecting both parties from damages or liability.

The legal framework: RPAPL § 881

When voluntary agreements can’t be reached, New York’s Real Property Actions and Proceedings Law § 881 provides a formal path. RPAPL § 881 is a New York law that allows a property owner to petition the court for a license to enter an adjacent property to make necessary improvements or repairs that cannot be done otherwise.

This law allows you and your neighbor the chance to petition the court for access when it’s necessary for improvements or repairs. The court balances both sides’ interests and, if access is granted, sets fair conditions such as compensation and insurance coverage.

Refusing reasonable access can have consequences. If your neighbor denies entry and a court grants an access license under RPAPL § 881, continuing to block access could result in penalties or liability for project delays or damages. The key is communication—your rights, and your neighbor’s rights must both be respected.

Limitations and Responsibilities

A license agreement only covers what’s written. If access is granted for repair work, it can’t be used for new construction without written consent. Access can’t be transferred to another party, and it ends once the permitted work is complete. Any violation—such as staying beyond the agreed period or causing unnecessary disruption—can expose you to legal claims.

Because these agreements are so specific, it’s wise to start negotiations early, ideally six months before work begins, and work closely with your RDP or an attorney experienced in construction law.

Construction Timeline

The timeline for building an ADU on your property is determined by many factors including the type of ADU you plan to build, the characteristics of your property, and the scope of your project. As a homeowner, you can anticipate the construction process to take anywhere from one week to 12 months. This timeline considers the physical construction of your ADU only. The entire ADU process, including design, permitting, and general preparation and obtaining a Certificate of Occupancy, covered in previous chapters, can vary based on outside factors and decisions you make along the way.

The construction of your ADU can only begin after you have finalized details with your Registered Design Professional and received all necessary permits.

As a homeowner, you can typically expect the construction of your ADU to undergo seven major phases from demolition and site preparation to move-in.

• Demolition/Site Preparation
• Foundation
• Framing
• Mechanical, Electrical, and Plumbing (MEP)
• Drywall and Insulation
• Exterior Fit Out
• Interior Fit Out, Finishing Touches, and Inspections

Demolition/
Site Preparation

• Site demolition and site preparation are the first steps your selected contractor will take to start the ADU construction process.
• All demolition and site preparation work must be covered under an approved permit before construction begins. Your RDP will help file the necessary permits and coordinate with the Department of Buildings (DOB).
• Site preparation includes work like the removal of obstructing vegetation or debris, grading or leveling the landscape, and making necessary utility connections like water, sewer, electrical, gas and septic hookups.
• Demolition can include the removal of any existing structures obstructing the site, such as shed or garage. Demolition can also include the removal of any interior walls or elements if you are building a basement, attic, or interior ADU inside your existing home.

Foundation

Framing

Mechanical, Electrical, and Plumbing (MEP)

Drywall and Insulation

Exterior fit out

Interior fit out, finishing touches, and inspections

Inspections

Before you can occupy your ADU, several inspections will confirm that the work meets safety and code requirements. Your RDP or general contractor will schedule and coordinate these inspections.

As the owner, you must provide your email address to the DOB to receive correspondence, inspection updates, and final sign-off notifications for your project. This allows you to monitor progress and stay informed as work proceeds.

Common inspections include:

• DOB inspections for structure, plumbing, electrical, and fire safety.
• Utility inspections for water, gas, and electrical connections when applicable.
• Special inspections by third-party professionals for items such as energy code compliance, sprinkler systems, or foundation work.
• Final DOB inspection to close out all open permits and verify compliance with approved plans.

What to Look for as a Homeowner During Construction

Questions and conversation starters to consider as a homeowner

Schedule and Timeline

• When will construction begin?
• When is it anticipated to end?
• What time can I expect workers on site to work on a day-to-day basis?
• Are there times I should be away from home? Times I should be present?
• What do I do if construction stops?

Safety and Construction Practices on Site

• How and where will materials, tools, and equipment be stored after the workday?
• What measures will be taken by the contractor team to protect my existing property and possessions?
• Will heavy machinery be needed for my project; if so, when can I expect it to be on site?
• If construction takes place inside the existing home, what measures will be taken to keep dust and debris away from the occupied areas?

Points of Contact

• Who is my RDP point of contact?
• Who is my general contractor point of contact?

Quality Assurance & Legal Matters

• When should I contact a lawyer or pursue legal counsel?
• How will I know if my project has been mismanaged?
• Who do I call if I think it's being mismanaged?

As the property owner, you should focus on hiring a general contractor who is licensed and insured and has the proper safety certifications and training. .5 The general contractor is responsible for maintaining a safe job site and ensuring all workers have the require safety training under NYC law.

If you see unsafe conditions such as unsecured scaffolding, missing protective barriers, or debris blocking walkways, notify your contractor in writing as soon as possible. Keep a record of your communication. If the issue is not resolved, you can contact 311 or the Department of Buildings (DOB) for inspection or enforcement.

Preparing a unit for occupancy

Legal occupancy

After construction is complete and before anyone moves in, you must confirm that your ADU is legal to occupy. Every new building in NYC needs a Certificate of Occupancy (CO), which officially states how the space can be used for the designed purpose and who can live there. Interior and attached ADUs will also require an updated or amended CO for the existing home before they can be legally occupied. To help with the process, you can choose to hire a filing representative, also known as an “expeditor,” who can assist with submitting paperwork and tracking approvals through the Department of Buildings. Your RDP remains responsible for preparing and certifying drawings, managing technical filings, and completing the steps for your Certificate of Occupancy (CO) requirements on DOB NOW: Inspections.

Documents that should be on hand during the inspection include:

• Approved construction plans
• Building permits
• Previous inspection reports
• Any correspondence with the DOB

If the inspector identifies any violations, they must be addressed, and a re-inspection may need to be scheduled. Check out Certification of Occupancy requirements at Certificate of Occupancy - Buildings.

What resources are there for
new landlords?

Deciding what to charge for rent

The location of your apartment, size, and amenities will determine how much you can reasonably charge for rent while maintaining tenants. One way to start determining how much to charge for rent is to look up similar sized apartments in your area, also known as comps, and see how much rent is for those units. You can use the ADU for You cost estimator to compare different amounts of rental income to see what works for your budget.

Rent Increases

According to New York Real Property Law Section 226-C, in New York, landlords must provide tenants with advance notice if the rent will increase by 5% or more and the length of notice is required to be 30 to 90 days depending on the length of the tenancy or lease term.

Broker Fees

On June 11, 2025, the Fairness in Apartment Rental Expenses (FARE) Act took effect in New York City. This law prohibits brokers who represent landlords from charging broker fees to tenants. Landlords or their agents must disclose other fees that the tenant must pay in their listings and rental agreements.

Choosing your tenant and managing
the relationship

When a landlord, broker, or owner screens prospective tenants during the application process, it is important to distinguish between legitimate questions and discriminatory questions that could violate fair housing laws.

Housing providers have a right to set financial and credit qualifications, a process that provides some assurance that the applicant will potentially be a good tenant. Qualifications and inquiries must be applied equally to all applicants and must not be influenced by race, national origin, or other protected factors.

Screening may include:

• credit checks
• work and landlord references
• personal references
• questions about the number of people who will live in the unit
• criminal background checks, including the sex offender registry
• home visits
• interviews

After renting your ADU

Once you’ve selected a tenant, create a written lease agreement that clearly outlines rent, payment schedule, maintenance responsibilities, and other terms. New York State law requires landlords to provide tenants with written notice of essential information such as building registration, emergency contacts, and procedures for repairs. Post or share this information in an accessible location.

Once you’ve selected a tenant, create a written lease agreement that clearly outlines rent, payment schedule, maintenance responsibilities, and other terms. New York State law requires landlords to provide tenants with written notice of essential information such as building registration, emergency contacts, and procedures for repairs. Post or share this information in an accessible location.

Landlord responsibilities in NYC

Landlords must ensure that buildings are safe, clean and well maintained, in both common areas and in individual apartments. Landlords of ADUs also need to create shared access to the street from the rear yard. Among other responsibilities, owners must provide and maintain security measures, heat, hot and cold water, and good lighting.

Homeowners must comply with the New York City Housing Maintenance Code and register their property annually with HPD if it contains rental units. If the building is rent-stabilized, the owner must also register rents annually with New York State Homes and Community Renewal. Most ADUs in one- or two-family homes will not create a “multiple dwelling” under the New York State Multiple Dwelling Law (MDL), but owners must still ensure their property meets all applicable local housing and safety codes.

Understanding What Qualifies

Your basement or cellar must meet specific requirements:

• Existed before April 20, 2024, as a rental unit with sanitation facilities like a toilet, sink, and bathtub or shower
• Operable windows in every habitable room
• Owner-occupied building where you live as your primary residence, not as an investor managing the property remotely
• Located within the program area, outside flood zones and coastal risk areas

Only one basement or cellar apartment qualifies per building. If you own a two-family home with finished spaces in both the basement and cellar, you choose one. The program focuses on single- and two-family homes where the owner occupies part of the building, keeping this about homeowners, not landlords with multiple properties.

Finding
Your Team

You cannot do this alone. The city requires a Registered Design Professional (RDP) like an architect or engineer to shepherd your application through the process.

You’ll need:

• Registered design professional (architect or engineer) to certify existing conditions, design future improvements, and submit documentation at each milestone
• Licensed master plumber for the sprinkler system
• General contractor for window installations and egress construction

Choose your design professional carefully. They will be your partner for up to ten years, tracking deadlines and coordinating inspections. You can find more guidance on selecting an RDP in Chapter 5.

Applying
for your Authorization
for Temporary Residence

You have until April 20, 2029, to apply for your ATR. Your design professional submits plans showing what exists now and what you’ll build over time. Both the Department of Buildings and the Fire Department will inspect your space. They will look for immediate dangers like illegal gas work, which triggers a vacate order, an official directive from a government agency that requires occupants to immediately leave a building because it has been deemed unsafe or dangerous. But minor violations don’t stop your application.

Your application includes certifications:

• Design professional confirms window sizes, ceiling heights, and egress paths
• You certify that you live in the building and that the unit existed before the cutoff date
• Tenant status on April 20, 2024, must be noted as it matters for their right to return after renovations

Living with an Authorization
for Temporary Residence

Your tenant stays. They pay rent. Life continues. But you cannot turn this into a short-term rental. No Airbnb, no 29-day stays. This program exists to preserve affordable housing, not create tourist accommodation.

If your unit sits empty, you can rent it, but only to one family maintaining a common household. The city watches for overcrowding and illegal conversions. Keep your occupancy straightforward.

The first
three months

Within the first three months, you must install:

• Hardwired smoke and carbon monoxide detectors in every required location
• Water sensors and alarms—basements flood, and early warning systems prevent catastrophic damage
• Emergency signage in the format HPD specifies, telling occupants where to exit and whom to call
• Natural gas alarms (if you have gas service) in each habitable room, plugged into AC power with battery backup or pair one battery-operated alarm with one plug-in model
• Entrance identification sign (if your basement or cellar entrance isn’t visible from the street), meaning you need to mount a permanent sign with red letters at least 5 inches tall on a white background on the primary dwelling, stating: “Basement (or Cellar) ADU entrance is located on Exposure [#], as viewed from street-facing side (Exposure 1)”

Inspect gas alarms yearly, replace batteries, and document everything.

Notify your tenant about the city’s emergency alarm system. Give them the information, confirm they received it, and keep records. You’re building a paper trail that proves compliance.

Year 1

By the end of year one, you must complete:

• One-hour fire separations between the ADU and the rest of your house that serve as fireproof barriers, helping give occupants time to escape
• Radon and vapor intrusion testing that stays below Department of Health thresholds or install mitigation systems before continuing the process
• Sprinkler system applications are a must and not negotiable

Your design professional files the sprinkler paperwork. Within six months, your plumber will obtain the permit. Within two years, the system goes in, gets inspected by both Buildings and Fire, and receives sign-off.

You will need to maintain the sprinklers forever, including annual inspections, testing, and documentation. Budget for this ongoing expense.

Years 3 and 4

You’re improving livability now. File applications for emergency escape and rescue openings—the windows that save lives during fires. They must meet specific requirements:

• At least four-square feet of clear opening
• No dimension smaller than 22 inches
• Sills no higher than four feet above the floor
• Window wells for basement windows, carving light and air into spaces that have lived in dimness

Your contractor obtains permits, installs the windows, and schedules inspections. By year four, this work closes out.

Years 7 and 8

If you’re legalizing a cellar with more than half the clear height sitting below grade, you need to build a dedicated egress. This isn’t the existing staircase. This is a secondary exit path that meets current codes. Design takes time; construction takes longer, but by year eight, you’re done.

Basements skip this step. Their above-grade positioning gives them easier egress options.

Year 9

You’re almost done. Submit your application for a certificate of occupancy or temporary certificate of occupancy. Your design professional compiles documentation from the previous years, demonstrating that every milestone was met. The city reviews, inspects, and issues your CO if you’ve completed everything as needed.

This certificate transforms your ATR into permanent legal status. The unit no longer operates under temporary authorization. It’s an official part of your building’s occupancy.

When tenant
must relocate

Some work cannot happen with tenants in place, including sprinkler installation, window cutting, and fire separation construction. When you need your tenant to move temporarily, the city requires you to protect their right to return.

Your notification requirements

• Within five days of receiving your ATR, notify your tenant in writing
• Thirty days before work begins, send notice again
• Include: work schedule, ATR identification number, and a clear explanation of their right to return
• If they vacate due to an Order to Vacate, you have thirty days after they provide contact information to send notice

Tenant responsibilities to preserve return rights:

• Must vacate when construction starts (refusal without your written permission forfeits their rights)
• Must provide a mailing address, phone number, or email within ninety days of vacating

Before
they return

• Give at least sixty days’ notice of completion date
• Include lease terms or occupancy conditions, including rent
• Tenant has thirty days to respond, if they don’t it means they’ve waived their right to return

This is a temporary relocation with the guaranteed chance to live in the unit again. Handle it carefully, document everything, and honor your tenant’s rights. The city will check.

Staying
Compliant

The city can revoke your ATR. Three violations within one year, or any single violation of the core ATR requirements, and you will be violated. Violations come from inspections. Buildings or Fire can walk through at any time, and you must grant access. Refusing entry triggers violations.

When you receive a Notice of Violation with a Request for Corrective Action (RCA), you have sixty days to fix the problem and schedule reinspection. Pass the inspection, and you may continue the program. Fail, and you may face removal from the program or a vacate order.

The Costs

Budget realistically. You’re paying for design services, permits, construction, inspections, and ongoing maintenance. Sprinkler systems run thousands of dollars to install and possibly hundreds annually to maintain. Window installations, fire separations, and egress construction all carry costs.

Some homeowners qualify for financing or grants. Research city and state programs supporting ADU development. But don’t let the cost deter you from starting. Operating illegally carries its own expenses like fines, vacate orders, and the constant risk of enforcement.

Our Goal

New York City has thousands of basement and cellar apartments. Most exist informally, housing families who need affordable options in an expensive city. This program acknowledges this reality while pushing toward safety. Your participation makes one unit legal, one household secure, and one building compliant.

This is an opportunity to invest in your property’s future and your community’s stability. When the ten years conclude and you hold that certificate of occupancy, you’ve accomplished something tangible by turning informal housing into permanent, legal, safe homes.

Ancillary Dwelling Unit

In New York City, an ADU is a self-contained home that’s built on the same lot as another house. Think of it as an extra living space, like a rear yard cottage, a basement, or a garage apartment. Each ADU is a separate home. It has its own kitchen, bathroom, and entrance. An ADU is compact, measuring no more than 800 square feet and intended for residential use only.

Attached (building)

A building shall be considered “attached” when it abuts two lot lines other than a street line, or another building or buildings other than a semi-detached building.

Attached ADU

An attached ADU is an addition that abuts to and expands the footprint of your house. It might run beside the main building in the side yard or extend into the rear yard. This is a separate home with its own entrance that either shares a wall with the main building or sits directly beside it with its own fire-rated wall.

Attic ADU

An attic ADU is an apartment built within your attic that has enough headroom. It uses space under the roof without changing the footprint of your property. Attic ADUs specifically require automatic sprinklers.

Basement or Cellar ADU

A basement or cellar ADU is an apartment below the first floor. To qualify as a legal ADU, the basement and cellars must meet safety requirements for ceiling height, light and ventilation, separate access, and fire protection. Basements and cellars have their own different requirements for compliance under the Building Code and Appendix U.

A basement is partly below ground, with less than half its height below grade, while a cellar is mostly or completely below ground, with more than half its height below grade. Because of these differences, cellars have stricter requirements for light, air, and emergency egress. Both must also address waterproofing, vapor control, and fire safety to ensure they can be safely occupied as legal homes.

Building Code

Building code is a set of rules and regulations that govern the design, construction, and maintenance of buildings with the goal of ensuring the health, safety, and welfare of building occupants.

Building Envelope

The building envelope is defined by the enclosure that separates conditioned space and exterior space.

Building Footprint

The building footprint is defined by the area your structure takes up on the ground plane.

Detached ADU

A detached ADU is a separate building in the rear yard. This could be a newly built ADU, a conversion of an existing garage into an ADU, or a new ADU constructed above a garage. Detached ADUs offer great privacy and flexibility conversion.

Detached (building)

A “detached” building is a building surrounded by yards or other open areas on the same zoning lot.

Dwelling Unit

A single unit providing complete independent living facilities for one or more persons, including permanent provisions for living, sleeping, eating, cooking, and sanitation.

Flood Zone

A flood zone is a geographic area characterized by its risk of flooding.

Floor Area Ratio (FAR)

A zoning regulation and formula that determines how many square feet are allowed to be developed on a given property. The floor area ratio is the total floor area on a zoning lot, divided by the lot area of that zoning lot.

Greater Transit Zone

The Greater Transit Zone boundaries are used for several zoning rules. They determine where special rules for Qualifying Residential Sites are applicable and how non-residential parking is calculated in some zoning districts. Being located within the Greater Transit Zone may allow for more or different development opportunities.

Gross Floor Area

Gross floor area includes the total square footage of a built structure. This includes all floors and space taken up by walls, ducts, and stairwells.

Lot Line

A lot line is a boundary that divides one property from another, or from a street or any public space.

Neighbor Notification Affidavit

A notice provided to adjacent property owners that you plan to apply for and build an ADU on your property.

NYC Rulemaking Process

New York City creates legally binding rules using a public process called CAPA (City Administrative Procedure Act). First, an agency drafts a rule and posts it for public review. People can share comments online or during a hearing. The agency must consider this feedback before publishing the final rule. For ADUs, this process sets rules on where they can be built. It also outlines safety standards and explains how homeowners can get approval.

City agencies like the Department of Environmental Protection and the Department of Buildings have passed rules. These rules help regulate and ensure quality in the ADU process. You can read more about these rules on the NYC Rules website.

Permitting

Building permits are required before construction projects can begin in New York City. Permit applications must be submitted to the Department of Buildings for code compliance review and approval.

Preapproved Plan Library (PAPL)

The Pre-Approved Plan Library (PAPL) is a design resource library where you can browse pre-designed detached and attached ADUs from industry professionals. The ADU designs in the PAPL have been pre-approved by the NYC Department of Buildings, allowing you to fast track the permitting, approval, and design process associated with building an ADU.

Rear Yard

The rear yard is the open space at the back of your property measured 20 feet from your rear lot line. In some cases, the rear yard is measured 30 feet from your rear lot line depending on the height of the primary building or lot size.

Rear Yard ADU

A rear yard ADU is a freestanding home (detached ADU) that sits behind your main building or a home attached to your main building (attached ADU).

Semi-Detached (building)

When the primary dwelling unit abuts another one family home (think duplex).

Setback

A setback is the required distance a building must be offset from an existing building, property line, or street. Setbacks help provide adequate light and air to surrounding streets and open spaces. They also improve fire safety through increased spacing between building structures.

Single-Family Residence

A “single-family residence” is a building containing only one dwelling unit, and occupied by only one family. A single-family residence may additionally include an ancillary dwelling unit in the same or an ancillary building.

Two-Family Residence

A “two-family residence” is a building containing not more than two dwelling units, and occupied by only two families. A two-family residence may additionally include an ancillary dwelling unit in the same or an ancillary building.

Zero Lot Line (building)

When the primary dwelling unit abuts one of the property’s lot lines.

Zoning Amendment

Zoning Text and Map Amendments Zoning amendments, whether citywide or area-specific, adjust the Zoning Resolution or map to accommodate developments that align with city planning goals. Amendments are either text-based (changing the Zoning Resolution) or map-based (changing zoning designations). These changes affect all properties equally within the modified area.

Zoning District

Each zoning lot has an underlaying zoning district based on the context of the lot in the city. Zoning districts are most commonly differentiated by residential, commercial, and manufacturing uses.

Zoning Resolution

The Zoning Resolution is the law for building and development in New York City. Zoning is how a city splits land into zones. Each zone has its own uses and rules that must be followed. These rules regulate features such as the shape, size, and placement of buildings.

In New York City, a maximum of one ADU is allowed on a property designated as single or two-family home. Each single or two-family home lies within a unique zoning lot. The characteristics of the lot are used to determine if and where you can build an ADU on your property.