§ 50. Entrance halls. Every entrance hall in every multiple dwelling erected after April eighteenth, nineteen hundred twenty-nine, shall be at least four feet in clear width from the entrance to the first stair, and beyond that shall be at least three feet eight inches in clear width. If such an entrance hall is the only entrance to more than one flight of stairs, the required width of such hall shall be increased in every part, for each such additional flight of stairs, by one-half the width required for one flight of stairs.