Only some kinds. As of October 2026, basement, cellar and new backyard ADUs are prohibited on lots in the special flood hazard area, the 10-year rainfall flood risk area or the coastal flood risk area on DEP's maps. An above-grade ADU in a building that existed on or before December 5, 2024 can be allowed if its lowest floor is elevated.
Which flood maps count?
DOB's rule names three areas, and a lot in any one of them is affected:
- Special flood hazard area, the FEMA high-risk zone.
- Coastal flood risk area, on DEP's interim flood risk map.
- 10-year rainfall flood risk area, also on DEP's map, which covers inland blocks that flood in heavy rain.
The rainfall map is the one that surprises people, because it can apply far from any shoreline.
How high must an above-grade ADU be?
Under rule 1 RCNY 105-08, effective October 30, 2025, the lowest floor of the ADU must be:
- At the design flood elevation, in the special flood hazard area.
- At least 3 feet above the highest adjacent grade, in the coastal flood risk area outside the special flood hazard area.
- At least 2 feet above the highest adjacent grade, in the 10-year rainfall flood risk area outside the special flood hazard area.
What about my garage?
HPD's guidebook says a rebuilt or converted garage in a flood zone may be permitted if it is elevated to the design flood elevation. A new backyard ADU, by contrast, is not allowed in these areas.
Why did the city draw these lines?
Below-grade apartments are the most dangerous place to be in a flash flood. DEP Commissioner Rohit T. Aggarwala said when the program launched that "basement apartments can only be safe in places that aren't prone to stormwater flooding."
Sources, as of October 2026:
- NYC DOB, rule 1 RCNY 105-08 (effective October 30, 2025) and ADU FAQs.
- NYC DOB announcement, September 30, 2025.
- NYC HPD, ADU guidebook.