N.Y. Mult. Dwell. Law § 178
Where the stair and public halls are not provided on each story with windows opening to a street, yard or court, a skylight equipped with ridge ventilators having an opening of forty square inches or more shall be provided in the roof over the stair, except as otherwise provided in paragraph g of subdivision three of section one hundred seventy-one. The glazed roof of such skylight shall not be less in area than nine square feet nor less than two feet in its least dimension, provided the ventilating and glass areas in such skylight are not less than herein required; except that in dwellings converted before April eighteenth, nineteen hundred twenty-nine, where a skylight smaller than such prescribed dimensions was in existence on such date, no structural change shall be required, but a ventilating skylight fitting the existing skylight opening in the roof beams shall be deemed sufficient.
N.Y. Mult. Dwell. Law § 178