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Ancillary Dwelling Units

UPDATE: Applications are not currently being accepted until DOB and DEP Rules are promulgated. (last revised 08/12/25)

In December 2024, New York City enacted Local Laws 126 and 127 alongside the City of Yes for Housing Opportunity zoning text amendment to support the safe creation and use of Ancillary Dwelling Units (ADUs) in one- and two- family homes. These initiatives work together to expand housing options by allowing a homeowner to add a new ADU to an attic, basement, cellar, a free-standing building, or a building attached to the current building.

Local Law 126 enables the legalization of some pre-existing basement and cellar apartments in certain NYC areas that have been occupied unlawfully by establishing a procedure to gradually bring basement or cellar apartments into compliance with Local Law 127 in ten years. Local Law 127 establishes comprehensive requirements for zoning, construction, fire safety and occupancy, providing a clear framework to create more housing while protecting health and safety. The information below explains generally what these changes mean for members of the public who are interested in ADUs, including homeowners, tenants, and building professionals.

General Requirements

  • Only one ADU per each 1- or 2- family residence
  • The homeowner must reside on the zoning lot with the ADU
  • Maximum of 800sq ft of zoning floor area
  • ADUs require a separate entrance
  • A two-family building adding an attic, basement, cellar, or attached ADU will be treated as a three-family building subject to the Multiple Dwelling Law (MDL), unless the unit is separated by a fire wall
  • A two-family building with a pre-existing basement or cellar unit undergoing the legalization process will be subject to Building Code Appendix U and additional fire and safety rules and regulations in lieu of the MDL. See legalization program for more details.

Exceptions/Restrictions

With some exceptions, one- or two- family buildings can add an ADU. These exceptions include:

  • Backyard ADUs* are not permitted in:
    • Historic Districts
    • R1-2A, R2A and R3A districts outside of the Greater Transit Zone
    • Special Bay Ridge District west of Ridge Boulevard or south of Marine Avenue
  • Subgrade ADUs are not permitted in high-risk flood areas demarcated by FEMA’s Special Flood Hazard Area, the Coastal Flood Risk Area, and DEP’s 10-Year Rainfall Flood Risk Area.
  • LL126’s subgrade dwelling legalization is restricted to existing one- and two-family buildings in the program area. The program area includes:
    • Bronx community districts 9, 10, 11, and 12
    • Brooklyn community districts 4, 10, 11, and 17
    • Manhattan community districts 2, 3, 9, 10, 11, and 12
    • Queens community district 2.

*Conversions of existing garages to an ADU are not considered Backyard ADUs per Zoning 12-10 and are not restricted in those areas.

ADU Legalization

Under Local Law 126 of 2024, existing occupied basement and cellars in existing one- and two- family buildings that are in the program area and not in a flood risk zone may be converted into a legal habitable dwelling. The unit must exist prior to April 20, 2024.

Subgrade ADUs under the Legalization Program may continue to be occupied so long as they meet each milestone and update the Authorization for Temporary Residence after meeting each milestone. These buildings are required to comply with the compliance timeline outlined in Local Law 126, based on the date of initial Authorization for Temporary Residence (ATR) certification

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