The height provisions of this chapter shall not apply to church spires, belfries, and domes, air-conditioning and ventilating equipment, chimneys, radio, television and telecommunication antennas, bulkheads, elevator enclosures, water tanks or similar accessory structures occupying an aggregate of 20% or less of the area of the roof on which they are located, and further provided that such structures do not exceed the height limit by the less of 20% of the height limit or 10 feet. All structures shall meet all height limitations required by the Federal Aviation Administration. Nothing in this chapter shall prevent the erection above the height limitation of a parapet wall or cornice extending above such height limit not more than three feet.