New York City uses the term ancillary dwelling unit and sets technical conditions by unit type and relationship to the primary building. A basement apartment, cellar unit, above-grade unit, backyard ADU, and manufactured home can have different code, access, fire, and filing requirements. The Department of Buildings’ current ADU FAQs and ADU types guide are the starting points; neither replaces review of the current code and the property file.
Match the unit type to the right checks
| NYC ADU type | Technical distinctions described by DOB | Questions to confirm |
|---|---|---|
| Above grade inside a one-family home | Separate entrance and fire separation; attic units have a different sprinkler treatment from other above-grade units in this typology | Does the actual floor, entrance, occupancy, and code path match this classification? |
| Basement or cellar | Light and air, emergency escape, water sensors, fire separation, utility shutoffs, sprinklers, and radon/vapor controls | Is the unit a basement or cellar under the code, and do flood, pilot-area, or legalization conditions apply? |
| Detached backyard ADU | Separate-building treatment, sprinkler and access provisions, plus zoning and placement conditions | Is the site eligible, what access path is required, and which separation, flood, and utility rules apply? |
| Manufactured home used as an ADU | State or HUD certification does not replace Appendix U review or DOB-approved site connections | Is the unit certified, and have placement, foundation, utilities, fire safety, and access been approved locally? |
These categories summarize DOB’s current descriptions. Exact dimensions, assemblies, exceptions, occupancy classification, and approval remain subject to Appendix U, the Construction Codes, zoning, and the project filing. Do not use a table row as a construction specification.
Basement and cellar units need a separate status check
DOB’s FAQs describe a 7-foot minimum ceiling height for habitable basement or cellar rooms and a cellar ceiling height at least 2 feet above grade. The City’s subgrade ADU pilot program has separate area, prior-occupancy, safety-milestone, and legalization conditions. The current FAQ says ATR applications are not yet being accepted; check the live City page rather than assuming the program is open or that an existing unit qualifies.
For covered basement and cellar filings, DOB’s service update describes an approved Department of Health and Mental Hygiene Radon and Vapor Level Certificate submission beginning August 17, 2026. Confirm the current service notice, certificate criteria, filing type, and deadline with DOB and DOHMH. A filing requirement is not a guarantee that the certificate will be issued or that the unit is eligible.
Check classification, flood status, and filing path
DOB says Appendix U generally applies to ADUs, with a stated exception for certain basement or cellar ADUs created with a new one-family residence. The FAQs also describe different filing types depending on whether a unit is in the same building or detached or separated by a firewall. The correct route can depend on whether the building is classified as a one- or two-family home, whether an additional dwelling changes the occupancy group, and how the unit is connected.
The filing date and energy-analysis status also affect which edition of the NYC Energy Conservation Code applies. DOB’s 2025 NYCECC transition notice describes the March 30, 2026 change and conditions for some earlier filings. Use the focused NYC ADU energy-code transition guide to check the filing, energy analysis, later compliance changes, and historic-building status.
Subgrade and backyard units have specific flood-risk constraints, while above-grade units may have separate elevation requirements. Check the current DEP maps and zoning status for the property, then ask a registered design professional and DOB to confirm the applicable code path, filing, inspections, and certificate of occupancy. A pre-approved plan or manufactured-home label does not make a site eligible or preapprove the site connections.
Keep the result local
NYC’s Appendix U and filing paths do not establish a rule for another New York municipality. This guide also does not determine a property’s eligibility, legalization result, or safe occupancy. For exact fire separation, egress, access, radon, flood, and utility details, use the current DOB and DOHMH material for the specific typology and obtain project-specific professional and authority review.