Upon transfer of title to a new owner or execution and recording of a mortgage, or upon said building becoming vacant, no two-family dwelling or mixed occupancy containing two or more families and no multiple dwelling shall be occupied, in whole or in part, until the issuance of a certificate by the Commissioner of the Building Department that said dwelling conforms in all respects to the requirements of this code. The head of the Department may, on the request of the owner or his certified agent, issue a temporary certificate of occupancy in his discretion.