One projecting sign for each lot, street frontage of each building unit, provided that it shall be attached to the main wall of a building, it shall project horizontally not more than six feet therefrom, it shall be erected at a height of not less than 10 feet nor more than 30 feet above the ground or sidewalk, it shall not exceed 40 square feet in surface area and, if lighted, it shall be illuminated internally with white or blue lights only.