§ 54. Cellar entrance. There shall be a direct entrance to the cellar, or to the lowest story if there be no cellar, from the outside of every multiple dwelling erected after April eighteenth, nineteen hundred twenty-nine, except that in non-fireproof multiple dwellings erected after such date which are three stories or less in height and occupied by not more than two families on any story, any stair leading to such cellar or lowest story may be located inside the dwelling provided it is enclosed in fireproof walls and fireproof doors and assemblies with the doors self-closing, at both the level of such cellar or lowest story and that of the story above. No such outside entrance existing in any multiple dwelling on April eighteenth, nineteen hundred twenty-nine, shall be obstructed.