All buildings and structures shall be subject to height limitations specified in the Schedule of District Regulations, except spires, belfries, domes, cupolas, scenery lofts, masts, flagpoles, chimneys, flues, ventilators, elevator penthouses, water tanks, towers, antennas, skylights, bulkheads and similar decorative features and necessary mechanical appurtenances usually carried above roof level. The aggregate horizontal area covered by such features shall not exceed 20% of the area covered by the principal building. The excess height of any such structure shall not be greater than the shortest distance from such structure to any required yard, nor shall the total height exceed twice the building height limitation in the district in which it is located. The foregoing does not pertain to transmission towers and associated right-of-way widths which are to be determined by standards in the National Electric Safety Code. Agriculture buildings not intended for human occupancy shall not be subject to the horizontal area limitations.