Parapet walls shall not extend more than six (6) feet above the height limit. Flagpoles, chimneys, cooling towers, electric display signs, elevator bulkheads, penthouses, finials, gas tanks, grain elevators, stacks, storage towers, radio towers, ornamental towers, monuments, cupolas, domes, spires, standpipes and necessary mechanical appurtenances may be erected as to height in accordance with existing or hereafter adopted laws or codes of the City.