California Code, Health and Safety Code - HSC § 2860

If the property is owned by a person who is not a resident of the district, and is not in charge or possession of any person, and there is no tenant or agent of the owner upon whom service can be made, who can after diligent search be found;  or if the owner of record of the property cannot after diligent search be found, the notice may be served by posting a copy in a conspicuous place upon the property for a period of 10 days, and by mailing a copy to the owner of record addressed to his address as given on the last completed assessment roll of the county in which the property is situated, or, in the absence of an address on the roll, to his last known address.