§ 150. Cellar and basement stairs. 1. Except as otherwise provided in this section, there shall be no inside stair communicating between the lowest cellar or the lowest story, if there be no cellar, and the floor next above, but any stair communicating between such floors shall be located outside the dwelling and if enclosed shall be fireproof in a fireproof enclosure with fireproof doors and door assemblies, with the doors self-closing, at all openings.

2. This provision, however, shall not apply to any stair not extending through more than one story and leading from an entrance hall to the upper stories where the walls enclosing such hall are fireproof and unpierced except for openings to stairs, elevators, apartments, public reception rooms, professional offices and the outer air. In no event shall there be any other opening from such an entrance hall, and all apartments and such professional offices opening therefrom shall be entirely separated from any space within such dwelling used for any other purposes by fireproof walls which shall be unpierced except for means of egress into the entrance hall. The provisions of this subdivision shall not apply to openings which are protected with fireproof vestibules. Any such vestibule shall have a minimum superficial floor area of fifty square feet and its maximum area shall not exceed seventy-five square feet. It shall be enclosed with incombustible partitions having a fire-resistive rating of three hours. The floor and ceiling of such vestibule shall also be of incombustible material having a fire-resistive rating of at least three hours. There shall be two doors to provide access from the entrance hall and any other space not used for openings to stairs, elevators, apartments, public reception rooms and the outer air. Each such door shall have a fire-resistive rating of one and one-half hours and shall be provided with a device to prevent the opening of one door until the other door is entirely closed. One of these doors shall swing into the vestibule from the entrance hall and the other shall swing from the vestibule into space which it serves. Such vestibule shall also be equipped with sprinklers and with an independent exhaust duct having a minimum cross-sectional area of one hundred forty-four square inches for each one thousand cubic feet, or fraction thereof, of air content, and such exhaust duct shall not be connected with any other ventilating system.

3. In dwellings three stories or less in height which are occupied by two families or less on every story, any stair leading to the cellar may be located inside the dwelling provided it is entirely enclosed with fireproof walls and with fireproof doors and door assemblies at both the cellar level and the story above, with the doors self-closing.

4. The provisions of subdivision one shall not prohibit or apply to an inside stair extending from a space used for commercial purposes in the cellar or lowest story to a store on the story next above, provided such stair is of incombustible materials, has closed risers, is enclosed between the two lowest floors with walls having a three-hour fire resistive rating and has fireproof doors and door assemblies at top and bottom, with the doors self-closing, and provided such commercial space in the cellar or lowest story is completely enclosed with partitions having a fire-resistive rating of at least two hours and there is no opening between the store on the story next above and any entrance hall or other public hall or public portion of the dwelling.