72 Pa. Stat. § 5020-410
All unseated lands within this Commonwealth, held by individuals, copartnerships, associations or corporations, either by improvement, warrant, patent, or otherwise, shall, for the purpose of raising county rates and levies, be valued and assessed in the same manner as other property.
Whenever any tax levy is made upon land as unseated, which the assessors for the same year by error or mistake returned assessed as seated, while the same ought or might legally have been assessed as unseated, such levy shall be deemed valid and regular for all intents and purposes, notwithstanding it differs from the copy of the duplicate furnished by the assessor. All records of the county commissioners charging lands as unseated with arrears of taxes shall be evidence of an assessment. No clearing over by mistake shall ever be deemed sufficient to render land seated.
72 P.S. § 5020-410