An apartment is illegal in New York City when the building is not legally allowed to have it. The record that decides this is the certificate of occupancy, which states how many dwelling units a building may have and what each floor may be used for. Two rules cause most of the trouble: a cellar can never be a legal dwelling, and going from two households to three turns a private dwelling into a multiple dwelling, with a different body of law attached. And if you are the tenant of an illegal unit — this is the part to hold on to — you still have rights.
How to check, in one step
Pull the building's certificate of occupancy and compare it with what is actually there. It will state the occupancy classification, the number of dwelling units permitted, and what each floor is approved for. If it says three apartments and there are five, the extra two are the question. If it is silent about the lowest level being residential and someone is living down there, that is the answer.
Certificates are public and filed against the building. Newer ones live in DOB NOW and older ones in the legacy system, which is why looking in only one place is the commonest reason people wrongly conclude a building has none — see DOB NOW vs BIS. And a building put up before 1938 may legitimately have no certificate at all, because that is when the city started issuing them; the owner can obtain a Letter of No Objection instead. The full explanation is in what is a certificate of occupancy.
The cellar rule
New York distinguishes a basement — at least half its height above curb level — from a cellar, which has more than half its height below it. A basement may be a legal apartment if it satisfies the requirements for light, air, ceiling height and a way out, and the certificate accounts for it. A cellar cannot, and no amount of finishing changes that: the problem is not the fittings, it is that there is not enough building above ground to put a window or a second exit in. HPD's own rule of thumb is that if a cellar has windows at all, they are usually too small for an adult to climb through. The distinction is set out at cellar vs basement.
The third apartment changes everything
Under the Multiple Dwelling Law a building occupied as the home of three or more families living independently is a multiple dwelling. Crossing that line brings with it annual HPD registration, the full weight of the Housing Maintenance Code, and fire-safety, egress and common-area requirements scaled to the number of households. Converting a basement in a two-family house into a third unit is therefore not a construction question with a legal footnote — it changes what the building is, and it requires a new certificate of occupancy. See multiple dwelling.
What the signs look like
None of these proves anything on its own. Together they are worth checking the certificate over.
- The unit is in a cellar, or a basement reached only through a boiler room or a shared corridor.
- The only window is small, high, or opens into a shaft; there is one way out.
- The apartment number is improvised — 1R, 2-rear, "the garden unit" — and appears on no buzzer panel.
- Rooms were made by partitioning, with a bedroom that has no window of its own.
- Utilities are shared or sub-metered informally, and the landlord asks for rent in cash without a written lease.
- The building has an open DOB violation for work without a permit. Those are public: the city's file currently carries around 13,000 active work-without-permit violations, alongside roughly 23,000 that were dismissed.
Your rights if the apartment is not legal
This is the section people most need and least expect.
- You are still a tenant. The illegality is the owner's problem with the city. It does not turn you into a trespasser or strip you of tenant protections, and a landlord cannot lawfully evict you by simply changing the locks or cutting services. Any eviction goes through housing court.
- The warranty of habitability still applies. It is implied in every residential lease, written or oral, and cannot be waived — including in an apartment that should not exist. See warranty of habitability.
- A vacate order does not end your lease. If the city orders the unit vacated, you generally keep the right to return once the order is lifted. See what a vacate order means.
- Relocation help may be available anyway. Occupants displaced by an HPD order to repair or vacate may be eligible for relocation services and temporary emergency housing whether or not the vacated unit was legal. HPD's Emergency Housing Services is the number to call.
- Rent already paid is not automatically forfeit, and what a landlord may collect going forward can be affected by the unit's status. This is genuinely fact-specific and is a question for a tenant attorney, not for a website.
The safety part, which is the real reason any of this exists
The rules about windows, ceiling heights and second exits are not paperwork. Occupants of illegal basement and cellar apartments face carbon monoxide exposure, inadequate light and ventilation, and — the one that kills — no way out in a fire. If you live in a unit like this, whatever else you do: test the smoke and carbon monoxide alarms, know your second route out and whether it is real, and do not store anything in the path of it.
Reporting one
Illegal conversions go to the Department of Buildings, through 311. You can report anonymously. Be specific — the address, which floor, how many units you believe there are, and what the exits look like — because a vague complaint gets a vague inspection. The mechanics are in how to file a complaint.
One honest caution, since this guide is read by tenants as often as by neighbours: reporting the apartment you live in can lead to a vacate order against your own home. That may still be the right thing to do, and the relocation protections above exist for exactly that situation — but go in knowing it, and take advice first if you can.
Before you sign
The certificate of occupancy, the number of legal units, the year built and the building's violation history are all public records, and they answer "is this a legal apartment" before you have paid anyone a deposit. You can pull them for any New York City address free at Check a Building. See also security deposit rules and how to check a building's violations.
This guide is general tenant education based on public records and city agency guidance, not legal advice, and it is not tenant screening. Whether a specific unit is legal turns on that building's own records, and the rights of a tenant in an illegal unit are fact-specific — consult a tenant attorney or a legal services organisation. Sources: NYC DOB — Certificates of Occupancy; NYC HPD — Basements and Cellars; New York State Multiple Dwelling Law article 1; NYC HPD — Orders (Order to Repair/Vacate) and Emergency Housing Services; NYC Open Data — DOB Violations (3h2n-5cm9), counts live on the date queried.