§ 170-a. Notwithstanding any other provision of this chapter, a
dwelling, other than a frame 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
all the provisions of this article are complied with and provided
further that

1. No part of such dwelling shall be used for business purposes.

2. No part of such dwelling shall be arranged, designed for or
occupied by roomers, boarders or lodgers.

3. The public hall or stair partitions and soffit of stairs shall be
fire-retarded and said partitions fire-stopped. The apartment entrance
doors and door assemblies in such hall shall be fire-proof and
self-closing.

4. The required stairs shall comply with the provisions of subdivision
three of section fifty-two of this chapter.

5. No door opening shall be permitted from a garage within the
building to the stair hall or public hall. However, any other opening to
the residential portion shall be provided with a fire-proof self-closing
door, and the door assemblies shall be fire-proof.

6. The cellar shall not be used for any purpose other than household
storage and mechanical equipment or appliances, and the cellar ceiling
shall be fire-retarded.