In case of a one-and-one-half-story (expansion attic area) dwelling, no such dwelling shall be hereafter erected unless the ground floor area contains not less than 1,400 square feet, exclusive of garage, car park, carport, open porches or terraces or any other unheated extensions outside the body of the house; provided, however, that a maximum of 200 square feet of expansion attic area, as hereinafter described, may be used and applied to the area requirements of the ground floor, diminishing said ground floor area to not less than 1,200 square feet.