No person shall carry out any exterior alteration, restoration, reconstruction, demolition, new construction or moving of a landmark or property within a historic district, nor shall any person make any material change in the appearance of such property, its light fixtures, signs, sidewalks, fences, steps, paving or other exterior elements visible from a public street or alley which affect the appearance and cohesiveness of the historic district, without first obtaining a certificate of appropriateness from the Historic Preservation Commission.