Wash. Rev. Code § 15.08.080
Personal service of said notice shall be made upon the person in possession or in charge of said premises or property if possible. If such person is not the owner, or personal service cannot be made on such person, then a copy of the notice shall be mailed or telegraphed to the owner at his or her home or post office address if known or can with reasonable diligence be ascertained. If personal service cannot be made upon any person in possession or charge of the premises or property and the name and address of the owner thereof are not known or cannot be so ascertained, then the notice shall be served by posting the same in some conspicuous place on the premises where the property to be disinfected or destroyed is situated, which service by posting shall be construed to be constructive personal service upon such owner. If the name and address of the owner are not known or cannot be so ascertained, service upon the person in possession or charge of the premises or property shall constitute substituted personal service upon the owner, in the absence of fraud or gross neglect.
RCW 15.08.080