New York City’s Zoning Resolution calls an accessory dwelling unit an “ancillary dwelling unit.” For a single- or two-family residence, the definition limits the ADU to 800 sq ft and allows no more than one per residence. A backyard ADU must have direct access along a side lot line through a side yard or open area at least 5 ft wide. The zoning lot must also be the owner’s primary residence when the first ADU is occupied. Separate district, flood-risk, and Landmarks-designated historic-district limits can prevent or change the ordinary pathway.
Match the building form to the height rule
For an ancillary unit in a detached, zero-lot-line, or semi-detached building, the listed limit is one story and 15 ft at any level. Where accessory parking is below the ADU, the building may instead reach two stories or 25 ft above adjoining grade, whichever is less; the height includes the apex of a pitched roof.
A free-standing building containing an ADU is limited to one-third of the rear yard or rear-yard equivalent and must be at least 5 ft from a side or rear lot line. Separately, the minimum distance between a residence and an ancillary dwelling unit on the same zoning lot is 10 ft. The lot-line setback and building-separation values describe different measurements and both may matter.
Verify district and project conditions
These values come from selected Zoning Resolution definitions and provisions for lots with two or fewer dwelling units. They are not a complete zoning determination. Check the current district rules, zoning-lot boundaries, flood status, historic-district status, proposed building configuration, adjoining grade, and the applicable code version. A zoning measurement does not determine fire access, fire separation, egress, building-code compliance, or permit approval. Have a qualified New York City design professional confirm measurements and route building and fire questions to the responsible authorities.