There shall be no enclosed usable space under stairways in an exit enclosure (enclosed stair shafts serving three or more stories), nor shall the open space under such stairways be used for any purpose.
EXCEPTION: Space under exterior stairways and interior stairways shall not be used for any purpose except when the usable space under stairs is enclosed and walls and soffits are protected on the enclosed side as required for one-hour fire-resistive construction in accordance with Chapter 9 of the Los Angeles Municipal Code (Building Code).