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. 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, but any building which was erected, altered or converted prior to July first, nineteen hundred fifty-five, to be occupied by such members or personnel or is so occupied on such date shall not be subject to the requirements of this code only so long as it continues to be so occupied if there are local laws applicable to such building and such building is in compliance with such local laws. A multiple dwelling does not include (i) a hospital, convent, monastery, asylum or public institution; or (ii) 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."
(a) A class A multiple dwelling is a multiple dwelling which is occupied, as a rule, for permanent residence purposes. This class shall include tenements, flat houses, maisonette apartments, apartment houses, apartment hotels, bachelor apartments, studio apartments, duplex apartments, kitchenette apartments, garden-type maisonette dwelling projects, and all other multiple dwellings except class B multiple dwellings. (b) A garden-type maisonette dwelling project is a series of attached, detached or semi-detached dwelling units which are provided as a group collectively with all essential services such as, but not limited to, water supply and house sewers, and which units are located on a site or plot not less than twenty thousand square feet in area under common ownership and erected under plans filed with the department on or after April eighteenth, nineteen hundred fifty-four, and which units together and in their aggregate are arranged or designed to provide three or more apartments.