Upon application for a special exception, the Zoning Hearing Board may approve the expansion or alteration of a use of land or buildings which is not in conformance with the provisions of this chapter, provided such expansion or alteration of use is restricted to an additional area not exceeding 35% of those existing buildings, structures, parcels, lot or tracts of land devoted to the nonconforming use and existing on the effective date of this chapter or any amendment thereto creating the nonconformity. However, such approved expansion or alteration of a nonconforming use shall not create new dimensional nonconformities except upon the approval of a variance by the Zoning Hearing Board.