No building hereafter enlarged or extended, or altered, wholly or in part, so as to change its classification or occupancy, and no building hereafter altered for which a certificate of occupancy has not been heretofore issued, shall be occupied or used, in whole or in part, until a certificate of occupancy is issued by the Building Official certifying that the work for which the permit was issued has been completed in accordance with the provisions of this Building Code; provided, that if the occupancy or use of such building was not discontinued during the work of alteration, the occupancy or the use of such building shall not continue for more than thirty days after the completion of the alteration, unless such certificate is issued.