A multiple dwelling is 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.

On and after 7/1/1955, a multiple dwelling shall also include residential quarters for members or personnel of any hospital staff which are not located in any building used primarily for hospital use provided, however, that any building which was erected, altered or converted prior to 7/1/1955, to be occupied by such members or personnel or is so occupied on such date shall not be subject to the requirements of the applicable chapeter of the Multiple Dwelling Law only so long as it continues to be so occupied provided there are local laws applicable to such building and such building is in compliance with such local laws.

A multiple dwelling shall not be deemed to include a hospital, convent, monastery, asylum or public institution, or a fireproof building used wholly for commercial purposes except for not more than one janitor's apartment and not more than one penthouse occupied by not more than two families. For the purposes of this chapter multiple dwellings are divided into two classes: Class A and Class B.