The maximum heights set forth in this chapter for buildings in all districts shall not apply to the erection of church or school spires or belfries, chimney flues, radio and television antennas, water tanks, stand pipes, bulk heads, stage towers or scenery lofts or similar parts of a building, provided that the height of that portion of the structure above the building shall not exceed 100% of the height of the main building, and provided further that the aggregate horizontal area of such part shall not exceed 20% of the ground area covered by the main building.