Section 50. Entrance halls

Every entrance hall in every multiple dwelling
  erected after April eighteenth, nineteen hundred twenty-nine,  shall  be
  at  least four feet in clear width from the entrance to the first stair,
  and beyond that shall be at least  three  feet  eight  inches  in  clear
  width.  If  such  an entrance hall is the only entrance to more than one
  flight of stairs, the required width of such hall shall be increased  in
  every  part,  for each such additional flight of stairs, by one-half the
  width required for one flight of stairs.