In New York City these are not two words for the same room. A basement is a storey partly below curb level but with at least half its height above it. A cellar has more than half its height below curb level. The line runs through the middle of the wall, and it decides something that matters enormously: whether the space can ever legally be somebody's home.
The definitions
| Basement | Cellar | |
|---|---|---|
| Height above curb level | At least one half | Less than one half |
| Height below curb level | Less than one half | More than one half |
| Can be legal living space? | Possibly, if requirements are met | No |
| Counted as a storey? | Generally yes | Generally no |
Height is measured from finished floor to finished ceiling, and "curb level" means the level of the street outside — which is why an identical room can be a basement on a sloping street and a cellar on a flat one. A practical rule of thumb from HPD: if a cellar has any windows at all, they are usually too small for an adult to climb through. That is not a coincidence; it is the same geometry that makes the space a cellar.
Why a cellar cannot be an apartment
Because the things that make a dwelling safe are exactly the things a cellar cannot have enough of: natural light, ventilation, ceiling height, and — above all — a way out in a fire. HPD is blunt about the risks of illegal conversions: carbon monoxide poisoning, inadequate light and ventilation, and inadequate egress. A basement may be legal living space if it meets the requirements for light, air, ceiling height and egress and the building's certificate of occupancy accounts for it. A cellar cannot be made legal by meeting requirements, because it cannot meet them.
What happens when the rule is broken
Illegal basement and cellar apartments are among the most consequential violations in the city's records, and the enforcement is correspondingly serious. Occupants of an illegal basement or cellar apartment may be ordered by the city to vacate. That is a genuinely difficult outcome for a tenant who has done nothing wrong, which is why it is worth knowing before signing rather than after.
Two things soften it. A vacate order does not terminate your lease, and if you are displaced by one you may be eligible for relocation assistance through HPD — including where the vacated unit was not legal. See what a vacate order means.
How to tell what you are looking at
The honest answer is that you cannot always tell by eye, and that the authoritative record is the building's certificate of occupancy, which states what each floor may legally be used for. If the certificate accounts for three apartments and there are four, the fourth is the question. If the certificate is silent about the lowest level being residential, that is the answer. See TCO vs Certificate of Occupancy and, for the fuller explanation, what is a certificate of occupancy.
A building erected before 1938 may have no certificate at all, which is ordinary rather than suspicious — but it means the legal use has to be established another way, through a Letter of No Objection.
Conversions, and the pilot
The city has run a limited Basement Apartment Conversion Pilot Program aimed at bringing some existing basement units up to a legal standard in defined areas. It is narrow, and it does not make cellar apartments legal. Converting a basement in a two-family house into a third unit is also not merely a construction question: it can change the building's status to a multiple dwelling, which triggers a different body of law and requires a new certificate of occupancy.
Checking before you sign
The certificate of occupancy, the unit count and the building's violation history are all public records. Check a Building pulls them together for any address, free — which is the cheapest way to find out whether the apartment you are about to rent is one the building is allowed to have.
Sources: NYC HPD — Basements and Cellars; NYC DOB — Key Project Terms: Basement/Cellar Apartments and Local Law 49 of 2019. General information about public records and city requirements, not legal or construction advice. Whether a specific space is legal turns on the building's own records.