Each permitted use may have one wall sign, provided that the sign shall not exceed an area equal to 5% of the area of the facade upon which it is erected, or 50 square feet, whichever is smaller. In computing permitted sign area, only one face of a building may be used as the principal face. In the case of a use located on the ground floor of a multi-story building, only the first floor facade area shall be used for the purposes of calculating the permissible sign area. In the case of a multi-occupancy structure, the allowable sign area shall be distributed among occupants having street level frontage. Single tenant structures shall use the first floor facade area for the purposes of calculating the permissible sign area.