One (1) sign bearing the brand or trade name of the station, of a design specified by the manufacturer, permanently affixed to the building or its own metal substructure, such sign not to exceed one-hundred (100) square feet in area on each side, which sign, if on its own metal substructure, shall be rigidly and securely attached to the ground surface so as to create no danger to life or limb and which sign, whether affixed to a building or on its own substructure, shall not exceed twenty-five (25) feet in height overall. Provided, that no such sign shall be so affixed or erected until permission in writing therefor shall have been issued by the Construction Code Official;