6. A private dwelling is any building or structure designed and occupied for residential purposes by not more than two families. Private dwellings shall also be deemed to include a series of one-family or two-family dwelling units each of which faces or is accessible to a legal street or public thoroughfare, if each such dwelling unit is equipped as a separate dwelling unit with all essential services, and if each such unit is arranged so that it may be approved as a legal one-family or two-family dwelling.
7. 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."