Any office, commercial or industrial building over two stories in height having a gross floor area of less than 10,000 square feet shall be permitted one freestanding sign, provided that the area of such sign shall not exceed 40 square feet. Any office, commercial or industrial building having a gross floor area of at least 10,000 square feet shall be permitted one freestanding sign, provided that the area of such sign shall not exceed 50 square feet. Where more than one use is conducted in a building or attached buildings on the same lot, only one freestanding sign shall be permitted.