72 Pa. Stat. § 5020-510
It shall be the duty of the several elected and appointed assessors in each of the two years succeeding the triennial assessment to give notice to the taxable inhabitants in like manner as after the triennial assessment, but in the following cases only; namely, in the case of real property, where buildings or other improvements have been newly erected or have been destroyed, and when coal, ore, or other minerals assessed under the triennial assessment have been mined out, since such triennial assessment; and in the case of personal property, offices, professions, trades and occupations, where there has been any alteration in the assessment, occasioning a different valuation from the former year, and also where persons have come to inhabit in the county since such triennial assessment.
72 P.S. § 5020-510