53 Pa. Stat. § 25003
In any city of the second class which has enacted, or may hereafter enact, a zoning ordinance, the provisions of the said ordinance affecting the required yards or courts for tenement-houses shall be the only effective provisions for such open spaces. In any city of the second class in which the open spaces about a tenement-house are not regulated by a zoning ordinance, the following provisions for tenement-houses shall apply.
Every tenement-house hereafter erected, altered or occupied shall have an open space, at the rear or the side or sides, equal in area to at least twenty per centum of the entire area of the lot upon which said tenement-house is erected, which open space shall be unobstructed by any over-hanging structure, except cornices or fire escapes; unless, such tenement-house is erected upon a corner of two streets neither of which is less than twenty feet in width, in which case said tenement-house shall have an open space attached to it, at 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-house is erected, which open space shall be unobstructed by any overhanging structure, except cornices and fire escapes; and any such tenement-house which shall be erected upon a lot bounded on three sides by streets not less than twenty feet in width, may be erected to cover the entire area of said lot, provided that every window opening from the living rooms in said tenement-house, shall open upon one of said streets. Any open space at the side or rear of such tenement-house shall be at least eight feet in width throughout its entire length. No court or open space between tenement-houses, or between wings of a tenement-house, shall be less than ten feet in width. If such tenement-house shall be built upon a lot which is bounded upon two opposite sides by streets, at least one end of every such open space shall abut upon one of such streets. Every court or shaft erected for the purpose of furnishing light or air to any such tenement-house shall open on one side into a street or into the yard or other open space, except such shafts as may be necessary for the ventilation of water-closets or bathrooms.
53 P.S. § 25003