Section 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.