7 Pa. Code § 137b.87
If enrolled land undergoes separation, and one of the tracts created through separation is converted to a use other than agricultural use, agricultural reserve or forest reserve within 7 years of the date of the separation, or is converted so that it no longer meets the requirements of section 3 of the act (72 P. S. § 5490.3), the owner of the ineligible tract owes roll-back taxes and interest with respect to all of the enrolled land. The ineligible tract may no longer receive preferential assessment under the act. The remaining enrolled land shall continue to receive a preferential assessment.
Example: Landowner A owns 100 acres of enrolled land, which is in agricultural use. Landowner A sells Landowner B a 50-acre portion of this enrolled land. Both 50-acre tracts continue in agricultural use, and preferential assessment continues with respect to both tracts. Six years after the original 100-acre tract of enrolled land was separated, Landowner B converts his 50-acre tract to industrial use. Landowner B owes roll-back taxes and interest with respect to the entire 100-acre tract. Landowner A's 50-acre tract continues to receive preferential assessment, and preferential assessment of Landowner B's 50-acre tract ends.
7 Pa. Code § 137b.87