72 Pa. Stat. § 5020-411
The elected and appointed assessors of the several counties shall, on seated lands, make the assessment in the county in which the mansion house is situated, when county lines divide a tract of land. Whenever the dividing line between two counties shall pass through the mansion house of any tract of land, the owner of the land so divided may choose as the situs of assessment either of the counties, by a written notice of his election to the commissioners of both counties. The elected or appointed assessors of the county so chosen shall assess therein all the tract of land. In the event that the owner shall refuse or fail to so choose, then the county in which the larger portion of the mansion house is situated shall have the right of assessment.
72 P.S. § 5020-411