No building or structure hereafter enlarged or extended, or so altered, wholly or in part, as to change its classification or occupancy, shall be occupied or used, in whole or in part, until a certificate of occupancy shall have been issued by the building official certifying that the work for which the permit was issued has been completed in accordance with the provisions of the law; provided, that if the occupancy or use of such building was not discontinued during the work of alteration, the occupancy or use of said building or structure shall not continue for more than thirty (30) days after completion of the alteration unless such certification shall have been issued.

SOURCE: GC § 31034.