53 Pa. Stat. § 14756
Such regulations, restrictions and boundaries may, from time to time, be amended, supplemented, modified or repealed. In case, however, of a protest against any such change, duly signed and acknowledged by the owners of twenty (20) per cent. or more either of the area of the lots included in such proposed change, or of those immediately adjacent in the rear or adjoining on the sides thereof extending one hundred (100) feet therefrom, or of those directly opposite thereto extending one hundred (100) feet from the street frontage of such opposite lots, such change shall not become effective except by the favorable vote of three-fourths of all the members of the council of such city. The provisions of the previous section relative to public hearings and official notice shall apply equally to all changes or amendments.
53 P.S. § 14756