As of October 2026, DOB allows basement and cellar ADUs with eligible one- and two-family homes, outside mapped flood risk areas. Habitable rooms need at least 7 feet of ceiling height. Cellar units carry extra requirements, including two independent ways out, emergency escape openings and a sprinkler system throughout the unit.
What is the difference between a basement and a cellar?
HPD's guidebook explains it by depth: a basement has less than half its height below grade, and a cellar has more than half below grade. For a cellar ADU, DOB also requires the cellar ceiling to be at least 2 feet above grade.
What does DOB require for a cellar ADU?
Under DOB rule 1 RCNY 105-08, effective October 30, 2025:
- Two independent means of egress, plus emergency escape and rescue openings.
- An automatic sprinkler system throughout the ADU under NFPA 13D as modified by the code. If the building has three or more stories, the whole building must be sprinklered.
- Required yards or open space outside required windows that sit at least 6 inches below the windowsill.
- A sign with red letters at least 5 inches tall at entrances not visible from the street.
What safety equipment is required below grade?
Every habitable room in a basement or cellar ADU needs water sensor alarms near flood entry points, with an AC-powered and a battery unit or one AC unit with battery backup, at a minimum of 75 dBA. The owner must inspect the alarms and replace batteries at least once a year, before a tenant moves in and after a flood. DOHMH certification for radon and certain chemical vapors is also required.
Where are basement and cellar ADUs not allowed?
Nowhere in DEP's mapped flood risk areas: the special flood hazard area, the 10-year rainfall flood risk area and the coastal flood risk area. See the flood zone guide.
Is a two-family house different?
Yes. DOB's FAQs say a basement or cellar ADU in a two-family home triggers R-2 classification and Multiple Dwelling Law requirements, because the building then holds three units. In an existing one-family home it can follow Appendix U of the Building Code. We factor this in early.