Proper service of the notice shall be by personal service, residence service or by certified mail; however, the notice shall be deemed to be properly served if a copy thereof is sent by certified mail to the last known address. lf any of the parties cannot be located or his address cannot be ascertained, the notice shall be deemed to be properly served if a copy is placed in a conspicuous place in or about the building or structure affected by the notice. If the notice is served by registered mail, the thirty day period within which the owner is required to comply with the order of the Building Inspector shall begin on the date the receiver received the notice according to the post office.
(1973 Code §150.10)