The hereinbefore specified height limits shall not apply to transmission or aerial towers, masts, flagpoles or monuments, nor to domes, cupolas, church spires, belfries, chimneys, water tanks, elevator penthouses, scenery lofts and similar parts of a building, provided that the aggregate horizontal area of such parts shall not exceed 20% of the ground area covered by the main building; nor shall the specified height limits apply to parapet walls not over four feet high.