N.Y.C. Admin. Code § EBC D505: D505 Spaces under stairs (MDL 151).

D505.1 Enclosed spaces.

It shall be unlawful to have a closet of any kind under any public stair. The space beneath every stair from the foot of the stair to a point where the soffit is 4 feet (1219 mm) or more above the floor shall be entirely enclosed with unpierced 1-hour fire barriers. Such enclosure beneath a flight of public stairs leading from the entrance story to the story next above shall be fireproof.

D505.2 Unenclosed spaces.

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.