72 Pa. Stat. § 5452.13
The proper assessors shall, between the triennial assessments, revise any assessment or valuation according to right and equity by correcting errors and by adding thereto any property, improvements or subjects of taxation which may have been omitted or any new property, improvements or subjects of taxation which may have come into being since the last triennial assessment. Any property, improvements or subjects of taxation which may have been omitted shall be assessed and made subject to taxation for the period during which said property, improvements or subjects of taxation shall have been omitted but in no event to exceed the period of five calendar years preceding the year in which the property, improvements or subjects of taxation omitted is first added to the assessment roll. Any such assessments as are made pursuant to the provisions of this paragraph shall be subject to appeal in the same manner as other assessments made pursuant to this act. Taxes levied on any such assessment shall not be made subject to the payment of any interest and penalties otherwise provided by law, except as the same are computed from the date of assessment made pursuant to this section. No bona fide purchaser of any property or subject of taxation without knowledge that the property or subject of taxation was omitted from assessment for purposes of taxation shall be subject to any taxation based upon the additional assessment made pursuant to this section.
They shall also add thereto the names of any persons who may have moved into such district and strike therefrom the names of any persons who have removed from such districts since the last triennial assessment.
The proper assessors shall also revise assessments and valuations between the triennial assessments by increasing or decreasing the same where the value of the property or subjects of taxation assessed or valued has changed by reason of any change of conditions thereon or adjacent thereto or in the vicinity thereof, or for the reason that the property assessed or valued has been subdivided or laid out into a plan of lots or other subdivisions, or for the reason that improvements have been placed thereon or added thereto, or for the reason that any public or other improvement has been made adjacent thereto or in the vicinity thereof, or for the reason that the assessor and the majority of the board decides that the assessor erred in the value which he placed on the property or subjects of taxation when making the triennial assessment, or where, for any other reason whatsoever, the value of the property has changed and it seems to the board necessary and equitable to make a change in the valuation thereof. The assessors shall also, between the triennial assessments in all cases where it is apparent that any assessment is not in accord with the generality or uniform standard of assessments, revise and correct the same by increasing or decreasing the same where the value of the property or subjects of taxation assessed do not conform to the generality or uniform standard of assessments.
No land assessed as acreage or unimproved property, which is subsequently laid out in residential lots and the plan of such lots is recorded, shall be assessed in excess of the total assessment of the land as acreage or unimproved property until such time as the lots are actually improved with permanent construction of any new building and either sold to a bona fide purchaser or occupied for residential purposes. Each such lot as sold or occupied shall be subject to reassessment beginning with the date of such sale or occupancy, and new construction thereon shall be subject to reassessment as provided above. When a department or agency of the Commonwealth or a municipality has ordered a sewer connection ban because of a lack of adequate sewage treatment facilities, the real estate affected by the order shall be reassessed for the duration of the order. The reassessment shall be based on the value of the best use of the land during the period of the reassessment. New single and multiple dwellings constructed for residential purposes and improvements to existing unoccupied dwellings or improvements to existing structures for purposes of conversion to dwellings, shall not be valued or assessed for purposes of real property taxes until (1) occupied, (2) conveyed to a bona fide purchaser, or (3) thirty months from the first day of the month after which the building permit was issued or, if no building permit or other notification of improvement was required, then from the date construction commenced. The assessment of any multiple dwelling because of occupancy shall be upon such proportion which the value of the occupied portion bears to the value of the entire multiple dwelling. As used in this paragraph, the word "dwellings" means buildings or portions thereof intended for permanent use as homes or residences and the phrase "affected by the order" shall be defined as the application for a building permit and the denial to the applicant of permission to proceed with the building or construction because of a sewer ban order.
All assessments required to be made by the proper assessors in the year between the triennial assessment shall be returned to the board not later than the first Monday of September of the year preceding the one for which it is made.
72 P.S. § 5452.13