The size of the total floor area of the accessory dwelling (excluding attic and cellar) shall not exceed 1,800 square feet. If the existing principal dwelling is listed in the National Register of Historic Places or meets the National Register Criteria as determined by the Bureau for Historic Preservation of the Pennsylvania Historical and Museum Commission, the Board of Supervisors may, at the applicant's request, deem the existing principal dwelling to be the accessory dwelling under the terms above and may waive the maximum square footage requirement in this Subsection C(3)(c), provided that the following standards are met: The accessory dwelling and associated historically significant outbuildings cannot be reconstructed, altered, restored, demolished or razed until a certificate of appropriateness has been issued in accordance with the procedures specified in § 122-120 and Chapter 61, Historic Districts.