In addition to the signs mentioned in this subsection, signs may be painted upon or attached to the exterior commercial structure, provided that it or they may not exceed in total over 10% of the ground floor plan (with a maximum of 3,000 square feet in any event) actually devoted to the business described in such sign, and provided further that a sign may be more than five feet in height if its width does not exceed five feet. No sign thus arranged vertically shall exceed 12 feet from the bottom of the sign.