The height limitation of this chapter shall not apply to church spires, belfries, cupolas, penthouses, and domes, not used for human occupancy; nor to chimneys, ventilators, skylights, windmills, water tanks, bulkheads, similar features, and necessary mechanical appurtenances usually carried above the roof level. Such features, however, shall be erected only to such height as is necessary to accomplish the purpose they are to serve and shall not exceed in cross-sectional area 20% of the ground floor area of the building.