Nothing in this subchapter shall be construed to prevent the ordinary maintenance or repair of any exterior architectural feature in the historic district which does not involve a change in design, material, color, or outer appearance thereof; nor to prevent the construction, reconstruction, alteration, restoration, or demolition of any such feature which the building inspector, or similar agent, shall certify is required for the public safety because of an unsafe or dangerous condition; nor to prevent the construction, reconstruction, alteration, restoration, or demolition of any such feature under a permit issued by a building inspector, or similar agent, prior to the effective date of the establishment of the historic district.