FAQs: Ancillary Dwelling Units

An ADU is an additional, self-sufficient dwelling unit not exceeding 800 square feet that is allowed in addition to a one- or two-family home. NYC zoning and building code requirements apply.

LL126 establishes a legalization program for pre-existing basement and cellar apartments in certain NYC areas. LL127 sets design and construction requirements for all ADUs, including new units and units being legalized.

Only one ADU is allowed per each one- or two-family home on the zoning lot, however there can be no more than 1 ADU construct on each tax lot. For example, if two one-family homes are on a zoning lot, each having its own tax lot, there will be a total of two ADUs. Another example is when there is a two-family home on the zoning lot, one ADU may be added.

Both laws were effective on June 16, 2025.

ADU filing may begin after DOB rules are published and Department of Environmental Protection (DEP) flood maps have been promulgated, which outlines specific flood areas where certain ADUs are prohibited.

ADUs are only allowed where zoning permits residential use and where there are no other zoning restrictions, including prohibitions in historic districts, certain low-density districts, and flood risk areas defined in DEP maps.

According to the Zoning Resolution, a backyard ADU is a structure developed after December 5, 2024, where the only dwelling is the ADU. Garage conversions do not qualify as Backyard ADUs.

No. They are prohibited in special districts like Bay Ridge, in R1-2A, R2A, R3A districts outside the Greater Transit Zone, flood-risk areas and historic districts.

It depends on the type of ADU. Certain types of ADUs, i.e. basement, cellar, and backyard ADUs are prohibited in the 10-year rainfall flood risk area, coastal flood risk area, and special flood hazard areas as defined in DEP maps.

It must be in the program area, occupied before April 20, 2024, contain acceptable kitchen and sanitation facilities, and assessed by city agencies to be deemed eligible.

Bronx Community District 9; Bronx Community District 10; Bronx Community District 11; Bronx Community District 12; Brooklyn Community District 4; Brooklyn Community District 10; Brooklyn Community District 11; Brooklyn Community District 17; Manhattan Community District 2; Manhattan Community District 3; Manhattan Community District 9; Manhattan Community District 10; Manhattan community district 11; Manhattan community district 12; and Queens Community District 2, and such other community districts as may be authorized pursuant to section 289 of the multiple dwelling law.

It allows eligible basement/cellar units within the program area to be temporarily occupied before obtaining a final CO, provided they meet safety milestones during a 10-year compliance period.

Submit an application to DOB for Authorization for Temporary Residence (ATR). Applications must be submitted by April 20, 2029.

Yes. Most pre-existing violations can be deferred and potentially waived if the property complies with the program milestones.

ATR may be revoked, and enforcement actions (i.e., vacate orders) may be reinstated.

Yes. Tenants in place as of April 20, 2024, have a right of first return after legalization renovations are completed. Stay tuned for a right of return rule by HPD.

Appendix U is created specifically to address the construction requirements for ADUs, including legalization ADUs. It modifies specific sections of the NYC Construction Codes. If a specific issue is not addressed in Appendix U, it must comply with all applicable requirements in the NYC Construction Codes.

Yes. All ADUs must have a separate entrance either from the exterior or a public corridor within the primary dwelling.

No. However, ADUs must have separate controls, disconnects, and shutoffs for heating, cooling, electrical, and gas systems. Water supply can be shared but must have separate shutoffs. Engage a design professional and see Appendix U for details.

Do ADUs need emergency escape windows?

Yes. All habitable rooms in ADUs must comply with emergency escape and rescue opening requirements per BC 1030 and FC 1025.

Yes, attic ADUs are allowed but must meet all BC Appendix U requirements, including sprinkler requirements. Appendix U also allows for option to comply fully with the NYC Construction Codes.

According to the NYC Zoning Resolution, an ADU located in the rear yard may not exceed 1 story or 15 feet. For ADUs above a garage in the rear yard, the height limit is two stories and 25 feet.

Yes. For legally existing attached garage and a garage within the primary home, an ADU may be created and parking may be omitted if permitted by zoning resolution. For a detached garage, an ADU may also be created. Note these are not considered Backyard ADUs by zoning definition. Additional elevation requirements will apply if it is within flood areas per DEP map.

Yes, new detached ADUs are allowed but are considered Backyard ADUs per zoning, and therefore prohibited in certain districts and flood areas as mentioned above. They must also comply with building separation distances, 5’ Fire Department access path requirements, and be fully sprinklered.

All habitable rooms must have at least 7 feet of ceiling height. Cellar ceilings must be at least 2 feet above grade.

intrusion mitigation is required before TCO or at the end of Year 1 for legalization sub-grade units.

Boilers must be enclosed with a 1-hour fire barrier, and the ADU must have a 1-hour fire separation from the primary dwelling.

Yes, but in existing 1-family homes only. In 2-family homes, basement or cellar ADUs outside the program area trigger R-2 classification and MDL requirements.

Within same building: Alt-CO. Detached or separated with firewall: NB with new CO and BIN.

Yes. Except for Legalization Program units, which must be reviewed by DOB’s Central Development Unit.

CO comment must state Ancillary Dwelling Unit per ZR 12-10 and BC Appendix U. Depending on the ADU configuration, the ADU will also be identified as i.e., ADU Apartment U per BC U202, ADU Apartment U per BC U202 and MDL Art 7-D, or ADU Apartment R. See Appendix U for specific instructions.

Yes. Starting Fall 2025, RDPs may submit generic plans for Backyard ADUs through DOB’s Office of Technical Certification and Research (OTCR). Approved designs will be posted in an online library of Pre-Approved Plans. Further information will be released on our website.