53 Pa. Stat. § 15173
In all suburban districts of cities of the first class, whether the premises concerned in said districts be assessed at rural, suburban, or city rates, wherever on improved premises a dwelling-house has been or may be erected that is detached or semidetached from the dwelling-house on the adjoining premises, and that is separated from the street on which it fronts by an intervening front yard, lawn, or space, no partition or division fence between such adjoining improved premises shall be required on or along that part of the boundary line that extends from the building line of the said detached or semidetached dwelling-houses to the sidewalk or pavement: Provided, That nothing in this act shall prohibit any occupant or owner of any such premises from erecting a division fence on or along such part of said boundary line, at his own cost: And provided, That no such fence shall be more than four feet high, without a permit therefor having been first obtained from the Bureau of Building Inspection.
53 P.S. § 15173