Height Limits. Except for one-and two-family dwellings as permitted in this Ordinance, penthouses or roof structures for the housing of stairways, tanks, ventilating fans, air-conditioning equipment or similar equipment required to operate and maintain the building, skylights, spires, cupolas, flagpoles, chimneys or similar structures may be erected above the height limits prescribed by this Ordinance but in no case shall such extension exceed 20 percent of the maximum height permitted in the district.