Ancillary Dwelling Unit (ADU)
City of Yes: Ancillary Dwelling Units (ADUs) – Summary
Under the City of Yes for Housing Opportunity, New York City significantly expanded where and how Accessory Dwelling Units (ADUs)—also referred to as ancillary dwelling units—are allowed, making it easier to add small, independent housing units within existing buildings or on zoning lots.
Core ADU rules (City of Yes)
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Where allowed: ADUs are permitted on the same zoning lot as a 1- or 2-family home (the City of Yes ADU framework).
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How many: 1 ADU max per 1- or 2-family residence on a zoning lot.
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Owner occupancy: At initial occupancy, the zoning lot must be the primary residence of an owner of that lot.
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Size cap: ADU is up to 800 sq ft of zoning floor area.
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Separate entrance: ADUs must have a separate entrance (from the exterior or a public corridor).
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Building/code compliance: ADUs must generally comply with NYC Building Code Appendix U (created specifically for ADUs), plus standard code items like egress and life safety.
What counts as an ADU type
City/agency guidance describes ADUs including:
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Backyard cottages (detached)
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Garage conversions (attached or detached)
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Basement / cellar / attic units (subject to specific code and timing limits—see exceptions below)
Exceptions + restrictions
1) “Backyard ADU” zoning limits
DOB’s FAQ notes that backyard ADUs are NOT allowed everywhere, including prohibitions in:
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Certain special districts (example given: Special Bay Ridge District)
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Some low-density districts in particular conditions (DOB FAQ references R1-2A, R2A, R3A outside the Greater Transit Zone)
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Flood-risk areas and historic districts (for backyard ADUs)
2) Flood area constraints
DOB FAQ highlights extra limits/elevation requirements for ADUs in flood-related mapped areas, and states backyard ADUs can be prohibited in flood-risk areas.
3) Basement vs. cellar rule complexity
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DOB FAQ explains Appendix U applies to all ADUs, except certain basement/cellar ADUs tied to new one-family construction, which instead follow other DOB rules—
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And DOB indicates new cellar ADU filings tied to new one-family homes were dependent on Housing Maintenance Code amendments taking effect.
4) Sprinklers / egress / safety
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Sprinklers: generally required in many ADUs; DOB FAQ calls out that attic ADUs must always be sprinklered.
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Emergency escape: habitable rooms must meet emergency escape / rescue opening requirements.
5) Height limits for rear-yard ADUs
DOB FAQ:
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Rear-yard ADU: max 1 story / 15 feet
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ADU above a garage in the rear yard: max 2 stories / 25 feet
6) “Backyard ADU” definition nuance
DOB FAQ: a backyard ADU is described as a structure developed after 12/5/2024 where the only dwelling is the ADU; and garage conversions are not “Backyard ADUs” by that definition (they may still be allowed—just categorized differently).
7) Multiple Dwelling Law trigger (important!)
DOB notes: If a two-family adds an attic, basement, cellar, or attached ADU, it may be treated as a three-family building subject to NYS Multiple Dwelling Law, unless separated by a fire wall.