The height limitations of this chapter shall not apply to church spires, fire walls, belfries, cupolas, chimneys, ventilators, skylights, bulkheads and 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 that they are to serve. Provisions of this chapter shall permit the erection of parapet walls or cornices for ornament without windows above the building height limit by not more than three feet. Quasi-public buildings and public buildings, schools and churches may exceed the height limit herein established, provide that such uses shall increase the front, rear and side yards one foot for each foot by which such building exceeds the height limit established for the district within which the use is located.