Permitted Exceptions. Height limitations stipulated elsewhere in this chapter shall not apply to church spires, belfries, cupolas and domes, monuments, chimneys, gas holders or water towers, smokestacks, elevator towers and enclosures, flagpoles, fire towers, or water tanks, steeples, (occupying in the aggregate less than 10% of the roof area where they are located) television aerials, or to parapet walls, except that no parapet wall may extend more than four feet above the limiting height of the building. Mechanical appurtenances such as condensors, exhaust fans, elevator housing and other similar equipment are exempt from these height restrictions provided they do not extend more than 15 feet above the maximum height limitation and are suitably screened. A mezzanine story shall also be considered to be a full story.