The height limitations of this chapter shall not apply to silos, church spires, belfries, cupolas, and domes not used or intended for human occupancy, or to municipal water towers or tanks. Chimneys, ventilators, skylights, water tanks, television and radio antennas and similar features, and necessary mechanical appurtenances usually carried on and above the roof level, are excluded from the maximum height shown in the Schedules of District Regulations if they do not exceed the maximum height by more than 15 feet. If such equipment is constructed as an accessory building or use, the height restrictions of this chapter shall apply.