Height exceptions. The height limitations of this chapter shall not apply to: church spires, belfries, cupolas and domes not for human occupancy; monuments, observation towers, transmission towers, chimneys, smokestacks, derricks, flagpoles, radio towers, masts and aerials, ventilators, skylights and water tanks; and necessary appurtenances usually carried above roof levels. Such features, however, shall be erected only to such height as is necessary to accomplish the purpose they are to serve, and for each foot by which the height permitted in the district is exceeded, the front, side and rear yards required by the district shall be increased an additional foot.