Wash. Rev. Code § 81.53.160
All notices required to be served by this chapter shall be in writing, and shall briefly state the nature of the matter to be inquired into and investigated. Notices may be served in the manner provided by law for the service of summons in civil cases, or by registered United States mail. When service is made by registered mail, the receipt of the receiving post office shall be sufficient proof of service. When, under the provisions of this chapter, it is necessary to serve notice of hearings before the commission on owners of private lands, property, or property rights, and such owners cannot be found, service may be made by publication in the manner provided by law for the publication of summons in civil actions, except that publication need be made but once each week for three consecutive weeks, and the hearing may be held at any time after the expiration of thirty days from the date of the first publication of the notice.
RCW 81.53.160