Section 54. Cellar entrance

There shall be a direct entrance to the cellar,
  or  to the lowest story if there be no cellar, from the outside of every
  multiple dwelling  erected  after  April  eighteenth,  nineteen  hundred
  twenty-nine,  except  that  in  non-fireproof multiple dwellings erected
  after such date which are three stories or less in height  and  occupied
  by  not  more  than two families on any story, any stair leading to such
  cellar or lowest story may be located inside the dwelling provided it is
  enclosed in fireproof walls and fireproof doors and assemblies with  the
  doors self-closing, at both the level of such cellar or lowest story and
  that  of  the  story  above.  No  such  outside entrance existing in any
  multiple dwelling on April  eighteenth,  nineteen  hundred  twenty-nine,
  shall be obstructed.