Section 151. Spaces under stairs

1. It shall be unlawful to have a closet
  of any kind under any public stair.
    2. The space beneath every stair from the foot of the stair to a point
  where the soffit is four feet or more above the floor shall be  entirely
  enclosed with unpierced fire-retarded partitions. Such enclosure beneath
  a  flight  of public stairs leading from the entrance story to the story
  next above shall be fireproof.
    3. Any unenclosed portion of the space  beneath  a  flight  of  public
  stairs  leading from the entrance story to the story next above shall be
  left entirely open and kept clear and free from encumbrance.