A multiple dwelling is, in the words of the New York State Multiple Dwelling Law, a dwelling "which is either rented, leased, let or hired out, to be occupied, or is occupied as the residence or home of three or more families living independently of each other." That threshold — three families, living independently — is one of the most consequential lines in New York housing law. Cross it and a different body of rules applies to the building.
Two families or three
A building housing one or two families is a private dwelling. At three it becomes a multiple dwelling, and with that come requirements that do not apply below the line, including:
- annual HPD registration, naming a head officer and a managing agent;
- the full weight of the Housing Maintenance Code and its inspection and violation regime;
- fire safety, egress and common-area requirements scaled to the number of households;
- heat and hot water obligations enforced as they are for apartment buildings.
This is why converting a basement in a two-family house into a third unit is not simply a construction project. It can change the building's status to a multiple dwelling, which triggers a different legal regime and requires a new certificate of occupancy from the Department of Buildings. It is one of the commonest ways an owner ends up in serious trouble without intending to — see cellar vs basement.
Class A and Class B
The law then divides multiple dwellings in two, and the distinction is about permanence rather than quality.
| Class A | Class B | |
|---|---|---|
| Occupied | For permanent residence purposes | As a rule transiently |
| Includes | Tenements, flat houses, apartment houses, apartment hotels, studio and duplex apartments, garden-type maisonette projects | Hotels, lodging houses, rooming houses, boarding houses, boarding schools, furnished room houses, club houses |
The statute's own words: a Class A multiple dwelling "is a multiple dwelling that is occupied for permanent residence purposes"; a Class B is one "which is occupied, as a rule transiently, as the more or less temporary abode of individuals or families who are lodged with or without meals."
Note the trap: this Class A and Class B have nothing to do with HPD's violation classes A, B and C. Same letters, entirely different scheme. One describes a building's occupancy type; the other grades the hazard of a condition. See HPD violation classes for the other one.
How many there are
The best public proxy for the number of multiple dwellings in New York City is the registration file, since registration is required at three units and above: roughly 203,000 registrations are on file. The registration itself is where the city keeps track of who is responsible for each of them.
A definitional detail worth knowing
The phrase is "three or more families living independently of each other," not "three or more apartments." Independence is the operative idea — separate households keeping their own homes, rather than the number of doors. It is also why the law extends to arrangements that do not look like an apartment building at all: rooming houses and lodging houses are multiple dwellings under Class B. The Multiple Dwelling Law also brought certain hospital staff residential quarters within the definition from 1955.
Where you meet the term
You will see "multiple dwelling" on registration records, in violation citations, in vacate orders, and in the name of programmes aimed at distressed buildings — HPD's Alternative Enforcement Program designates "severely distressed multiple dwellings" specifically. It is legal shorthand for "a building the city treats as housing more than a couple of households", and most of the enforcement machinery is built on top of it.
To see how a specific building is classified, along with its unit count, year built and registration status, search the address free at Check a Building.
Sources: New York State Multiple Dwelling Law, article 1 (definitions), as published by NYC DOB; NYC Open Data — Multiple Dwelling Registrations (tesw-yqqr). Counts are live totals on the date queried. General information about public records and statutory definitions, not legal advice.