Yes, ADUs are legal in NYC. The City Council approved the “City of Yes for Housing Opportunity” zoning text amendment on December 5, 2024, along with Local Laws 126 and 127, which together allow homeowners with one- or two-family homes in low-density residential districts to add a self-contained dwelling unit to their property.1NYC Department of Buildings. Ancillary Dwelling Units You still have to meet detailed zoning, building code, and safety rules, file through the Department of Buildings, and work with a licensed design professional. Some ADU types can be filed now; others are still on hold while the city finalizes its rules.
One terminology note before you start searching city websites: NYC calls these “Ancillary Dwelling Units,” not “Accessory Dwelling Units.” The rest of the country uses “accessory,” and state programs do too, but every NYC agency and filing system uses “ancillary.” Use the city’s term when searching the DOB site or filing through DOB NOW.1NYC Department of Buildings. Ancillary Dwelling Units
Which ADU Types Are Allowed
Only one ADU is permitted per property, and the configurations the city allows depend on your home and lot:
- An above-grade unit inside a one-family home, such as an attic conversion, new floor addition, or horizontal enlargement. A one-hour fire separation between the ADU and the primary home is required, and attic ADUs must have automatic sprinklers.
- A basement or cellar apartment in an existing one- or two-family home. Cellar ADUs need ceilings at least two feet above the surrounding grade.
- A third unit in a two-family home, placed behind a full concrete or masonry fire wall running from foundation to roof, with completely independent exits, fire protection, and building systems.
- A detached unit, meaning a backyard cottage or garage conversion built as a separate structure. Detached ADUs must be sprinklered, sit at least 10 feet from the main house and 5 feet from property lines, and have a clear access path at least 5 feet wide running from the street for fire access.
Every ADU has to function as a self-contained residence with its own kitchen, bathroom, sleeping area, and entrance.2NYC Department of Buildings. Ancillary Dwelling Unit (ADU) Types
Lot, Size, and Zoning Rules
Zoning allows ADUs on lots with one- or two-family homes in low-density residential districts, with some restrictions in flood-prone areas and certain neighborhoods.3NYC Department of City Planning. City of Yes for Housing Opportunity
A detached ADU is capped at 800 square feet of floor area, and its footprint cannot exceed one-third of the required rear yard. It must sit at least 5 feet from side and rear property lines and at least 10 feet from the main house. Converting an existing garage? Non-compliant setbacks from the original structure can generally be kept.2NYC Department of Buildings. Ancillary Dwelling Unit (ADU) Types
No additional off-street parking is required. If you convert a garage into an ADU, you can eliminate the parking space it occupied.
Safety and Building Code Requirements
Local Law 127 lays out construction standards under Building Code Appendix U. Habitable rooms in basements and cellars need a clear ceiling height of at least 7 feet, and cellar ADUs additionally require the ceiling to sit at least 2 feet above the surrounding grade plane.4New York City Administrative Code. Appendix U – Ancillary Dwelling Units
Every ADU needs proper plumbing, electrical, water, and sewer connections. Below-grade units require separate utility shutoffs, water sensors, radon or vapor mitigation, and emergency egress. Sprinklers are mandatory for all below-grade ADUs, all detached ADUs, and all attic conversions. The one carve-out: above-grade ADUs inside a one-family home (other than attic units) do not require sprinklers.2NYC Department of Buildings. Ancillary Dwelling Unit (ADU) Types
Where Basement ADUs Are Prohibited
Basement and cellar ADUs are flatly prohibited in three categories of flood-risk areas: FEMA-mapped Special Flood Hazard Areas, the 10-year rainfall flood risk area, and the coastal flood risk area.5New York City Administrative Code. U202.3 Prohibited Locations This is not a variance situation. If your property sits in any of these zones, a below-grade ADU is off the table. Check the city’s flood maps before you spend time planning a basement or cellar conversion.
How to Get an ADU Approved
Applications are filed through DOB NOW: Build, the Department of Buildings’ online system. You will file either an Alt-CO-GC job (for modifying an existing structure) or a New Building-GC job (for a detached unit).1NYC Department of Buildings. Ancillary Dwelling Units
You cannot file ADU plans yourself. All drawings must be stamped and signed by a New York State licensed architect or professional engineer, known as a Registered Design Professional. This applies to every ADU type. The city has started offering pre-approved ADU plans to reduce design costs and speed up review.6NYC Department of Housing Preservation and Development. ADU for You Introduction and DOB Pre-Approval RFS Guidelines
Once your permit is approved, the DOB inspects at various stages during construction. After the unit passes all inspections, you need a Certificate of Occupancy from the DOB to formally recognize it as a legal residence. Without the CO, the unit is not legally habitable, and you cannot rent it.
What Cannot Be Filed Yet
As of the time of writing, the DOB is not yet accepting applications for new cellar ADUs or for legalizing existing basement or cellar apartments. Those filings are on hold until the city finalizes amendments to the Housing Maintenance Code and promulgates the needed DOB rules.1NYC Department of Buildings. Ancillary Dwelling Units Above-grade conversions, detached new construction, and garage conversions can be filed through DOB NOW now.7NYC Department of Buildings. Service Notice: Ancillary Dwelling Units to be Filed in DOB NOW
Legalizing an Existing Basement Apartment
Local Law 126 creates a pathway to legalize some unpermitted basement and cellar apartments through a phased 10-year compliance program, but only in designated community districts in the Bronx, Brooklyn, Manhattan, and Queens.1NYC Department of Buildings. Ancillary Dwelling Units Eligible homeowners have until April 20, 2029, to apply for the pilot program.8NYC Rules. Establishment of Temporary Residence Program for Eligible Basement or Cellar Residences The 10-year window is meant to give owners a realistic runway for the upgrades needed to reach full code compliance: sprinklers, fire separation, egress, and ventilation.
Legalization filings are separate from new ADU applications and, as noted, are not yet open through DOB NOW. If you have a tenant in an unpermitted basement unit now, watch the DOB website for the announcement that legalization filings have opened.
Costs, Grants, and Taxes
Construction costs vary widely by ADU type. A basement conversion with minimal structural work costs far less than a detached backyard cottage built from scratch. NYC labor and material costs run high, and for an 800-square-foot detached unit you could easily be looking at $200,000 or more before design fees, permits, and utility connections. Budget separately for the licensed design professional, DOB permit and filing fees, utility hookups, and the sprinkler systems most ADU types require.
The Plus One ADU Program combines state and city support. New York State Homes and Community Renewal provides construction grants of up to $175,000 per homeowner, and the city’s Department of Housing Preservation and Development offers low-interest loans on top. To qualify, you must be an owner-occupant of a single-family property where zoning allows an additional unit, with household income at or below 165% of Area Median Income.9NYC Department of Housing Preservation and Development. Plus One Accessory Dwelling Unit Program Term Sheet The program also provides technical assistance with predevelopment and construction planning.10Homes and Community Renewal. Plus One ADU Program
Adding an ADU will raise your property taxes because the city reassesses your property’s value. NYC offers a partial property tax exemption for the first ten years after your ADU is completed, which you apply for through the NYC Department of Finance.11NYC Department of Housing Preservation and Development. NYC Plus One ADU Program Frequently Asked Questions Your existing homeowners policy will need updating once the ADU is finished; depending on your insurer and whether the unit is attached or detached, you may adjust your current policy or need a separate one.12NYC Department of Housing Preservation and Development. How to Use This Guidebook – ADU for You During construction, builder’s risk insurance is usually carried by the contractor, though some lenders or insurers require the homeowner to hold it, so confirm who covers what before work begins. If you plan to rent the ADU, ask your insurer about landlord or rental property coverage, since standard homeowners policies don’t cover tenant-related liability.