Height. No building shall exceed a maximum of three stories or 42 feet in height, whichever is the lesser, except that elevator enclosures, air-conditioning units and similar roof appurtenances may extend above the height limit, provided that they occupy an aggregate of not more than 10% of the area of the roof surface; and further provided that such structures do not extend above the roof surface by more than 10 feet. Unless designed as an integral part of the building, such structures shall be concealed from horizontal view by wall-type structures coordinated with the design of the building.