§ 9. Buildings converted or altered. 1. On or after December fifteenth, nineteen hundred sixty-one, no multiple dwelling shall be enlarged or its lot diminished so that the yard or other unoccupied areas shall be less in size or area than the minimum dimensions prescribed in section twenty-six.

2. A building not a dwelling, if converted or altered after April eighteenth, nineteen hundred twenty-nine, to a multiple dwelling, shall thereupon become subject to all the provisions of this chapter applicable to dwellings of like class and kind erected after such date.

3. A dwelling of one class or kind, altered or converted after April eighteenth, nineteen hundred twenty-nine, to another class or kind, except as hereinafter in this section and in articles six and seven otherwise provided, shall thereupon become subject to all the provisions of this chapter applicable to a building of that class or kind, erected after such date, to which it is altered or converted.

4. No dwellings shall be altered so as to be in violation of any provision of this chapter relating to dwellings of like class and kind erected after April eighteenth, nineteen hundred twenty-nine, except that it shall be sufficient for the purposes of this section that tenements shall comply with article seven, converted dwellings comply with article six, and lodging houses comply with section sixty-six. Nothing in this section shall, however, be deemed to prohibit the conversion or alteration of any multiple dwelling, other than a converted dwelling and a lodging house, from a class A to a class B multiple dwelling, or vice-versa, provided that the entire dwelling is of fireproof construction and is made to conform to the applicable provisions of section sixty-seven, and to all other provisions of this chapter applicable to multiple dwellings of like class and kind erected before April eighteenth, nineteen hundred and twenty-nine.

5. a. Any multiple dwelling may be altered to conform with any provision of this chapter applicable to dwellings of like class and kind and not expressly limited in application to dwellings erected after April eighteenth, nineteen hundred twenty-nine; or to conform to the provisions of this chapter relating to egress, or to exits from apartments, in dwellings of like class and kind erected after such date if such dwelling also conforms or is made to conform to all the requirements relating to stairs and public halls in dwellings erected after such date, except that existing dimensions of stair landings, treads and risers need not be changed.

b. An apartment in any part of a fireproof multiple dwelling erected before April eighteenth, nineteen hundred twenty-nine, may be altered or subdivided and need not conform to the requirements of paragraph a of this section relating to stairs and public halls provided each newly created apartment has access to a public hall which provides horizontal egress to at least two stairs. If both of such stairs are not arranged and designed as required by sections two hundred thirty-six and two hundred thirty-seven, at least one of such stairs shall be so arranged and designed, and the other shall be arranged and designed as required by section two hundred thirty-nine. Both stairs shall be protected by automatic sprinkler heads throughout.

6. Any tenement previously converted to other uses may be altered or reconverted to a tenement by complying with the provisions of article seven, provided, however, that its height and bulk were not enlarged prior to such alteration or reconversion except as permitted by and in accordance with the provisions of sections two hundred eleven and two hundred twelve of this chapter.

7. In any class B multiple dwelling, except a rooming house or lodging house, any apartment may be occupied as an apartment in a class A multiple dwelling, provided such apartment complies with all the requirements for apartments in class A multiple dwellings erected after April eighteenth, nineteen hundred twenty-nine. In any rooming house or lodging house, one apartment may be occupied as an apartment in a class A multiple dwelling, provided such apartment is occupied solely by the owner, janitor, superintendent or caretaker.

8. Any apartment in any class A multiple dwelling may be occupied for single room occupancy only if such dwelling complies with the provisions of section two hundred forty-eight and all other provisions of this chapter applicable to such dwelling.

9. Excepting a frame dwelling, any dwelling three stories or less in height erected after April eighteenth, nineteen hundred twenty-nine as a one or two-family dwelling may be converted to a multiple dwelling to be occupied by not more than three families in all, with a maximum occupancy of two families on each floor in a two story building and one family on each floor in a three story building, provided however that it shall be unlawful for any such dwelling converted at any time since October fifteenth, nineteen hundred fifty-two, to have any boarders or roomers. In each such instance, compliance shall be required with all the provisions of article six, including section one hundred seventy-a of said article.

10. If any class A dwelling erected before April eighteenth, nineteen hundred twenty-nine, is altered so as to increase the number of living rooms by more than twenty per centum, such dwelling, except as otherwise provided in sections two hundred eighteen, two hundred thirty-five and two hundred thirty-six, shall be made to conform to the requirements of this chapter with respect to class A dwellings of like class and kind erected after such date.