Adding an ADU in NYC: City of Yes Rules for Homeowners

Since City of Yes for Housing Opportunity took effect in December 2024, most NYC one- and two-family homes may add one accessory unit (zoning calls it an “ancillary dwelling unit”) of up to 800 square feet, in a backyard building, an attic, a basement or cellar, or an attached addition. The main limits: an owner must live on the lot when the unit is first occupied, it needs its own entrance, and basement units and backyard cottages are banned in flood-risk areas, while backyard units are also banned in historic districts and some low-density districts far from transit.

Below is what the zoning text and the Department of Buildings (DOB) actually say, what is excluded, how to file, and where the separate basement-legalization pilot fits in. Use it to decide whether a project is worth paying an architect to study.

What the zoning text allows

City of Yes for Housing Opportunity was adopted in December 2024, and the Zoning Resolution’s new ADU definitions are dated December 5, 2024. Under §12-10, an ancillary dwelling unit is “an additional dwelling unit, permitted on the same zoning lot as a single- or two-family residence that does not exceed eight hundred square feet of floor area.” Only one is allowed per single- or two-family residence on a lot.

DOB’s implementation page lists the possible locations: an attic, basement or cellar, a free-standing building, or a building attached to the existing home. A backyard ADU is one in a building developed after December 5, 2024 whose only dwelling unit is the ADU. It can stand alone or abut another building on the lot, and it must be reachable directly from a side yard or open area along a side lot line at least five feet wide.

Where ADUs are not allowed

One more limit from the definition: where backyard ADUs are banned, the rules don’t let you legalize a building that was already sitting in a required yard before December 5, 2024. Check your lot’s zoning district, historic-district status and flood maps before anything else. See zoning, FAR and air rights and flood insurance in NYC.

Comparing the ADU types

Attic, basement and attached ADUs still have to meet the flood-zone rule about building below the flood-resistant construction elevation. Height, yard and setback rules for a backyard building, and any parking questions, depend on your zoning district; your architect or engineer will check them against the Zoning Resolution.

Owner-occupancy and renting it out

At the time an ADU is first occupied, the lot must be the primary residence of an owner. The zoning text sets that condition at initial occupancy only. Once you have tenants you are a landlord, and lease, deposit and fee rules apply. See security deposit rules for landlords and rental income taxes. Use the rent out or sell calculator and cash flow calculator to see whether the rent covers the build cost.

How to get a new ADU approved

1. 1

   Screen the lot

   Confirm the building is a one- or two-family residence, and check the zoning district, historic-district status, and FEMA and DEP flood maps. Look up the lot and its certificate of occupancy on Roofy’s building lookup.
2. 2

   Hire a registered design professional

   An architect or engineer prepares plans that meet zoning, the Building Code and, for some two-family homes, the Multiple Dwelling Law. Historic-district work also needs Landmarks approval.
3. 3

   File in DOB NOW: Build

   Log in with an NYC.ID and create an Alt-CO-GC job (converting or adding to an existing building) or a New Building-GC job. On the PW1 tab, answer “yes” to the ADU question and give the ADU location, where its main entrance opens, and a pre-approved plan number if you are using one.
4. 4

   Get permits and build

   Work starts only after DOB approval and permits. See when you need a DOB permit.
5. 5

   Get a new certificate of occupancy

   The schedule of occupancy will describe the ADU. Don’t rent it until the CO covers it.

More on permits: do you need a DOB permit? and how to read a certificate of occupancy. Check your lot on Roofy’s building lookup.

The City says it opened the DOB NOW application portal for new ADUs in September 2025. DOB’s ADU page (last revised April 9, 2026) also says it now accepts applications for a one-family home built with a cellar ADU, under 1 RCNY §105-08(d)(4) and Local Law 80 of 2026. Ask your design professional whether that path fits your project.

Existing illegal basement apartments: the pilot

City of Yes covers new ADUs. A separate law, Local Law 126 of 2024, creates a pilot to legalize basement and cellar apartments that already exist in one- and two-family homes in certain community districts and outside flood-risk zones. Accepted owners can legalize without penalties, with a phased ten-year compliance schedule under an Authorization for Temporary Residence (ATR).

1. Before April 20, 2024

   Unit must already exist

   Only basement and cellar apartments in place before this date can enroll.

2. December 2024

   Local Laws 126 and 127 enacted

   Passed alongside City of Yes for Housing Opportunity.

3. Pending final rules

   Applications not yet open

   DOB says it is not accepting pilot ATR applications until its rules are promulgated.

4. November 10, 2025

   Proposed rules published

   DOB and HPD published proposed pilot rules, with public hearings in December 2025. Applications open only once final rules take effect.

5. Over 10 years

   Phased upgrades

   Accepted owners must meet compliance milestones, including safety work such as sprinklers and flood water sensors, while tenants can stay. Under Local Law 126, a tenant who must move out for the work has a right of first return.

6. April 20, 2029

   Owner application deadline

Before you spend money

• Ask your lender and your insurer how a new unit affects your mortgage terms and coverage before you build.
• Talk to neighbors if you share a driveway or the side access the backyard rule requires.
• Budget for professional fees, filing and inspections on top of construction.