Which type fits my property?
Start with the flood map and your building type. If your lot is in a mapped flood risk area, basement, cellar and new backyard ADUs are off the table, and above-grade options must be elevated. If your house is attached or a rowhouse, the ADU rules do not apply to it. From there, the choice comes down to what space you have.
What rules apply to every NYC ADU?
- One ADU per eligible one- or two-family home, up to 800 sq ft.
- The owner's primary residence must be on the zoning lot at initial occupancy.
- No new parking space is required.
- The ADU does not count toward the zoning lot's maximum number of units.
- Plans are filed with the Department of Buildings through DOB NOW: Build.
How do the types compare?
| Type | Key limits, as of October 2026 | Flood risk areas |
|---|---|---|
| Backyard ADU | 1 story, 15 ft; 5 ft from side and rear lot lines; 10 ft from the house; up to one-third of the required rear yard | Not allowed |
| Garage conversion | Existing structure; 2 stories and 25 ft if parking stays below | May be allowed if elevated to the design flood elevation |
| Basement or cellar | 7 ft ceilings; cellars need two exits and sprinklers | Not allowed |
| Attic | Always sprinklered; enough clear headroom | Elevation rules apply |
It depends on the details. Some special districts, historic districts and the R1-2A, R2A and R3A zones limit backyard ADUs. A lot can pass the flood test and fail on side-yard access. We run every check before we suggest a type.