72 Pa. Stat. § 5020-413
Whenever the dividing line between any township and city or borough, or between any two townships, as now or may be hereafter located, shall pass through the mansion house of any tract of land, the owner of the land so divided may choose, as the place of residence of its occupants, either of the townships or the borough, by a written notice of his election to the commissioners of the county. A choice once so made shall be binding on the owner and occupiers of such mansion house and on future owners thereof. In case of the neglect or refusal of the owner of such land to make an election as aforesaid, the persons occupying said mansion house shall be regarded as residing wholly within the township, and the elected or appointed assessors of such township shall, in such case, or when he elects to reside in the township, assess therein such persons, and all the tract of land on which such mansion house is erected.
72 P.S. § 5020-413