Required off-street parking areas for one- and two-family residences shall be located on the same lot as the principal building to be served. Under unusual circumstances, when approved as a special exception by the Zoning Hearing Board, parking areas may be located off-site, in all zoning districts, provided that the parking area is not more than 300 feet from the premises of the principal building or use to be served by such areas. Said distance shall be measured along public walkways from the nearest lot line of the lot on which the use is located to the nearest lot line of the lot on which the required parking is located.