ADUs in Flood Zones: What NYC Homeowners Need to Know Before They Build
If your home is near the coast or in a low-lying area, there's a good chance flood zones are going to come up in your ADU planning — and it's worth understanding what that means before you go any further.
NYC's new ADU rules are clear on this point: certain types of ADUs simply cannot be built in high-risk flood areas. It's one of the most important things to check before investing time and money into a project.
Which ADUs Are Affected?
The flood restrictions mainly apply to units that are below ground or at ground level:
Basement and cellar ADUs are prohibited in DEP flood risk areas
Detached backyard ADUs are prohibited in DEP flood risk areas
Garage conversions and above-grade units are generally less affected, but site-specific conditions still matter
How Do You Know If Your Property is in a Flood Zone?
The city published an online Flood Risk Area Map — created by the Department of Environmental Protection (DEP) — to define exactly where ADUs can and can't be built.
You can check your property's flood status by typing in your address on DEP’s flood risk mapper: nyc.gov/dep/floodriskmap
What If Your Property IS in a Flood Zone?
It doesn't necessarily mean you can't build an ADU at all. It means certain types of ADUs won't work. For example:
A basement ADU might be off the table
A new detached backyard ADU is not allowed, but a new ADU attached to a single-family home may work
A garage conversion might be feasible even where a new detached unit wouldn't
An attic ADU in a one-family home is generally unaffected by flood rules
The key is figuring out which options remain available for your specific property. As of today, there are no official channels through which property owners can dispute their property’s flood zone status.
Why This Matters
Flood zone rules aren't just bureaucratic hurdles — they exist because basement flooding is a real, serious risk in parts of New York City. Hurricane Sandy, and several storms since then, proved that. The city doesn't want people living in units that could flood. And frankly, neither do you.
Don’t Guess - Let Us Check
If you're not sure whether your property is in a flood zone, or you’re wondering what ADU options are still available if it is in a flood zone, we can help. A free assessment from Second Key will flag any flood-related issues right away.