Where permitted in commercial, office or industrial districts, be not more than 20 feet in height from the highest point of said sign, including supporting structures, to the average finished grade below said sign, nor, when located less than 25 feet from the property boundary or street line, be less than eight feet in height from the average finished grade below said sign to the lowest point of said sign, excluding vertical support structures, which vertical support structures shall not be more than one foot wide nor less than three feet in distance from each other; except that one monument-type freestanding sign not exceeding four feet in height and located not less than 15 feet from any property line, and not located as to impair visibility for motorists, shall be allowed.