"Floor area" means the sum of the gross horizontal areas of the floors of a building or buildings, measured from the exterior faces of exterior walls and from the centerline of division walls. Floor area shall include: basement space where the ceiling is at least thirty inches above the ground elevation of adjacent portions of the lot, elevator shafts and stairwells at each floor, mechanical equipment rooms or attic spaces with head room of seven feet six inches or more, penthouse floors, interior balconies and mezzanines, enclosed porches. Floor area shall not include: accessory water tanks and cooling towers, mechanical equipment or attic spaces with headroom of less than seven feet six inches exterior steps or stairs, terraces, breezeways, and open spaces. (Ord. 523 § 1(part), 1989)