53 Pa. Stat. § 14931
No building shall be occupied as a tenement unless it shall have appurtenant to it in the rear or at the side, and as a part of the lot upon which it is located, and open space equal to at least twenty per centum of the entire area of such lot, which open space shall be unobstructed by any overhanging structure except fire-escapes required by law; unless, however, such tenement shall be located upon a corner of two streets, neither of which is less than twenty feet in width, in which case said tenement shall have an open space attached to it, in the rear or at the side next the adjoining lot, equal in area to at least ten per centum of the entire area of the lot upon which said tenement is located, which open space shall be unobstructed by any overhanging structure except fire-escapes required by law; and any such tenement located upon a lot bounded on three sides by streets, none of which is less than twenty feet in width, may cover the entire area of said lot: Provided, That at least one window, not less than the minimum size hereinafter provided, opening from each of the rooms in such tenement, shall open upon one of the streets. Such open space attached to every tenement shall be at least eight feet in width throughout its entire length. No court or open space between tenements, or between wings of a tenement, shall be of a less width than twelve feet, except in buildings erected prior to June seventh, one thousand eight hundred and ninety-five, and which are not over three stories in height; where the open space is between the walls of the walls of the wings of tenements there shall be an approved open space five feet in width, provided the length of such wing or wings is not more than thirty feet, and, for each additional ten feet in length, there shall be an additional foot in width, provided there is at the rear or at the side of the lot upon which the building is erected an open space equal to at least twenty per centum of the entire lot area, which open space shall be unobstructed by any overhanging structure except fire-escapes required by law. If such tenement is situated on a lot which is bounded on two opposite sides by streets, then at least one end of every such open space shall abut upon one of said streets. Every court or shaft furnishing light or air to any such tenement shall open, upon one side, into a street, or into a yard or open space, except such shafts as are used solely for ventilation of water-closets or bath-rooms, which, for the purpose of cleaning same, shall have a door or window giving sufficient access for such purposes on the ground floor leading thereto. Buildings erected as tenements, or physically altered into tenements, prior to June seventh, one thousand eight hundred and ninety-five, shall be exempt from the provisions of this section governing the percentage of the lot occupied, the width between the wings and the width of the court; and such other buildings shall likewise be exempt as have been listed on the records of the Department of Public Health and Charities, and consecutively occupied as tenements prior to the passage of this act: Provided, however, That whenever any building which had been erected prior to January first, one thousand nine hundred forty-three, is about to be or has been converted into a tenement, the board of adjustment of any city of the first class authorized to be established by the act, approved the sixth day of May, one thousand nine hundred and twenty-nine (Pamphlet Laws, one thousand five hundred fifty-one), shall have the authority, after investigation and public hearing, to determine that the area of open space required under the terms of this section to be appurtenant to tenements is not necessary for the protection of the public health and welfare, and to issue a certificate of variance, as respects said open space. In such case, the provisions herein contained respecting the area requirement of open space shall not be applicable.
53 P.S. § 14931