Necessary appurtenant structures such as a church spire, belfry, cupola, dome, smokestack, monument, derrick, conveyer, flag pole, communications tower, mast, antenna, aerial, airplane hangar, roof tank, building service equipment, roof structure other than a penthouse, chimney or parapet wall, or any similar appurtenance, provided that the side yards, rear yard and front setback be increased one foot horizontally for each two feet that the height of such structure exceeds the height permitted in the district.