Height of structure. No structure shall be erected, converted, enlarged, reconstructed or structurally altered to exceed the height limit herein established for the district in which the structure is located, except that penthouses or roof structures for the housing of elevators, stairways, tanks, ventilating fans, solar energy systems, or similar equipment required to operate and maintain the structure, and fire or parapet walls, skylights, steeples, flagpoles, chimneys, smokestacks, electric and telephone service poles, water tanks, silos, storage hoppers, elevators or similar structures, if permitted in the district, may be erected above the height limits herein prescribed.