The height limitations of this chapter shall not apply to church spires, belfries, cupolas, penthouses and domes not used for human occupancy; nor to chimneys, ventilators, skylights, 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. The provisions of this chapter shall not apply to prevent the erection above the building height limit of a parapet wall or cornice for ornament (and without windows) extending above such height limit not more than five feet. Nonresidential buildings may exceed the height limitations of this chapter, provided that such uses shall increase the front, rear and side yards one foot by which the building exceeds the height limit herein established for such zone in which it is located, and further provided that in no case shall any building have a height greater than 50 feet.