53 Pa. Stat. § 24901
In all dwellings hereafter to be erected with a front of sixteen feet or less--providing they do not exceed sixty feet in length and thirty-five feet in height--the cellar or foundation wall shall not be less than eighteen inches in thickness, the front and rear walls not less than nine inches, and the party walls not less than nine inches. Walls binding on the street, lane or alley shall not be less than thirteen inches to the top of the first floor joist. In all dwellings over sixteen and not over twenty feet front nor more than forty-five feet high the foundation walls shall not be less than eighteen inches in thickness, the front walls shall not be less than thirteen inches, the party walls not less than thirteen inches, and for all dwellings over twenty feet front the party walls shall not be less than thirteen inches, and the front, rear and side walls shall not be less than thirteen inches to entire height. If there be a stone ashler used three inches thick or less it shall not be included in the thickness of any wall herein specified.
53 P.S. § 24901