Extension and/or expansion as permitted in Subsection D(1), (2), (3), (4) and (5) above shall be permitted only by special exception by the Zoning Hearing Board in accordance with the provisions of Article XVIII of this chapter, provided that all extension and/or expansion shall comply with all the standards of the district in which the use or building is located, including dimensional standards, parking standards, and all building coverage and impervious coverage limitations. Physical expansion of a nonconforming building or building housing a nonconforming use shall be permitted only one time and shall be limited to 25% of the gross floor area of the existing building. Expansion of a nonconforming use throughout an existing building shall also be permitted only one time and shall be limited to 25% of the gross floor area taken up by the existing nonconforming use.