Wash. Rev. Code § 4.28.120
If a party having or claiming a share or interest in or lien upon any property sought to be appropriated for public use be unknown, and such fact be made to appear by affidavit filed in the office of the clerk of the court, the notice required by law in such cases may be served by publication as in the case of nonresident owners, and such notice shall be directed by name to every owner of a share or interest in or lien upon the property sought to be so appropriated, and generally to all persons unknown having or claiming an interest or estate in the property or any portion thereof, and all such unknown parties shall in all papers and proceedings be designated as "unknown owners," and shall be bound by the provisions and be entitled to the benefits of the judgment the same as if they had been known and duly named.
RCW 4.28.120
Eminent domain: Title 8 RCW.
Publication of legal notices: Chapter 65.16 RCW.