16 Pa. Stat. § 1770.1
Whenever in any county there is any construction of a building or buildings not otherwise exempt as a dwelling after January first of any year, and such building is not included in the tax duplicate of the county, the authority responsible for assessments in the county shall, upon the request of the board of county commissioners, cause to be inspected and reassessed, subject to the right of appeal and adjustment provided by the act of Assembly under which assessments are made, all taxable property in the county to which major improvements have been made after January first, and to give notice of such reassessments in accordance with 53 Pa.C.S. § 8841(c) (relating to assessment roll and interim revisions) to the authority responsible for assessments, the county commissioners and the property owner. Such property shall then be added to the duplicate and shall be taxable for county purposes at the reassessed valuation for that proportionate part of the fiscal year of the county remaining after the property was improved. Any improvement made during the month shall be computed as having been made on the first of the month. A certified copy of the additions or revisions to the duplicate shall be furnished by the board of county commissioners to the proper tax collector for the county and, within ten days thereafter, the tax collector shall notify the owner of the property of the taxes due the county.
Whenever an assessment is made for a portion of a year as above provided, the same shall be added to the duplicate of the following or succeeding year unless the value of the improvements has already been included in said duplicate.
16 P.S. § 1770.1