As used herein, the term "gross floor area" shall include the total gross horizontal area of all floors of a dwelling measured to the exterior of the outside walls, including the horizontal floor area of any garage which is part of the dwelling, the horizontal floor area of any enclosed breezeway which is part of the dwelling, and the horizontal floor area of any enclosed porch, but excluding the following:
(1) The floor area of a basement or cellar shall be excluded in calculating the gross floor area, provided that the basement or cellar has more than 1/2 of its height (measured from floor to ceiling) below finished grade.
(2) The floor area of spaces with ceiling heights (measured from floor to ceiling) of less than five feet shall be excluded in calculating the gross floor area. The floor area of attic space (to wit, unfinished space between the ceiling joists of the top story and the roof rafters) shall be excluded in calculating the gross floor area.
(3) Any floor area under the roof of a roofed porch or similar roofed structure attached to the dwelling shall be excluded in calculating the gross floor area, provided that the roofed porch or similar roofed structure is not enclosed.