A certificate of occupancy (C of O) is an official NYC Department of Buildings document that states how a building may legally be used and occupied — for example, that it's residential with a specific number of dwelling units, and what each floor is approved for. It's the city's confirmation that a building matches its approved plans and is legal to occupy for the stated purpose.
What a C of O certifies
It spells out the building's occupancy classification and use (residential, commercial, mixed), the number of dwelling units permitted, what each floor may be used for, and any occupancy limits. This is why it matters for renters: if the C of O says three apartments and the owner carved it into five, the extra units may be illegal — affecting your rights and safety, since illegal units often skip required egress and fire protections. A basement or cellar apartment is a classic example of a unit often not legal under the C of O. New buildings, and existing buildings undergoing major alterations that change use, egress or occupancy, are required to obtain one.
TCO vs. final CO
A Temporary Certificate of Occupancy (TCO) means a building (or part of one) is safe to occupy but has items to resolve before a final certificate; TCOs are time-limited (commonly about 90 days) and must be renewed. New residential buildings often open under a TCO. A final C of O is permanent, issued once the completed work matches approved plans, all agency approvals are in, fees are paid, and paperwork is resolved. A building on a long-expired TCO can signal final sign-offs were never completed — sometimes minor, sometimes not — though an expired TCO alone isn't proof of a serious problem.
Why some buildings have no C of O at all
NYC began issuing certificates of occupancy in 1938. Buildings built before then generally aren't required to have one unless later alterations changed the building's use, egress, exits, or occupancy. For a pre-1938 building without one, the owner can request a Letter of No Objection (LNO) from the DOB borough office confirming the building's legal use. So finding no C of O on a century-old walk-up is often normal, not automatically a red flag.
How it connects to whether your unit is legal
The C of O (or, for older buildings, an LNO) establishes how many legal apartments a building has and what the space may be used for. If your apartment isn't accounted for in that legal use — an extra unit, a converted basement, office space rented as a home — it may not be a legal dwelling unit. This intersects with rent regulation and your rights; see am I rent stabilized? and how to file a complaint if you suspect an illegal conversion.
How to find a C of O
Records are public, tied to a building's address and BIN: DOB NOW: Public Portal (current system, newer certificates) and BIS (older and historical). Because certificates can live in either system and older buildings may have an LNO instead, Check a Building lets you look up a building's C of O status alongside its violations and complaints in one free search — helpful for confirming whether a unit is legal before you sign a lease. Property research and tenant education, not a background check. See also how to look up your NYC landlord and what a stop work order means.
Free tool for public NYC building records — tenant education, not tenant screening. Sources: NYC DOB — Certificates of Occupancy toolkit; NYC Business — Certificate of Occupancy.