Where aggregate floor area is the basis for establishing the amount of the permit fee, such aggregate floor area shall be calculated as follows:
(a) The aggregate floor area shall be the sum of gross horizontal areas of the several floors of the building or other structures, including interior balconies and mezzanines, if any, and including basements, cellars and useable parts of attics; except that attic floors in dwelling houses and rooming houses shall not be included in the aggregate floor area unless they are at least fifty percent used for habitable rooms, kitchens, bathrooms, toilet rooms or for other similar living purposes.
(b) All horizontal dimensions shall be taken from the exterior faces of walls, including walls or other enclosures of enclosed porches, dormers, penthouses and sidewalk vaults, and including the areas of open porches and open porticos. The measurements shall not include the areas of outer or inner courts, or the areas of outside terraces or steps, or similar open structures outside of the enclosing exterior walls.
(c) Whenever a story height of a building exceeds fifteen (15) feet measured as herein specified for "H", the aggregate floor area of such floor shall be deemed to be H/15 times the measured floor area thereof. "H" shall be the height measured floor level of such story to the floor level of the story above, or where such story is the uppermost story of the building and has a finished ceiling, shall be the height measured from the floor level of such story to a level one (1) foot above the average ceiling level within the measured floor area. Where no ceiling is provided in the uppermost story of a building and the roof is supported on girders or trusses, "H" shall be measured to the mid-height of such girders or trusses. With a sloping roof and no ceilings, "H" shall be measured to the mid-height of the sloping roof. With exposed roof joists on bearing walls "H" shall be measured to a level one (1) foot above the average level of the roof joists.