Wash. Rev. Code § 11.96A.140
Notwithstanding any other provision of this title, notice of a hearing does not need to be given to a legally competent person who has waived in writing notice of the hearing in person or by attorney, or who has appeared at the hearing without objecting to the lack of proper notice or personal jurisdiction. The waiver of notice may apply either to a specific hearing or to any and all hearings and proceedings to be held, in which event the waiver of notice is of continuing effect unless subsequently revoked by the filing of a written notice of revocation of the waiver and the mailing of a copy of the notice of revocation of the waiver to the other parties. Unless notice of a hearing is required to be given by publication, if all persons entitled to notice of the hearing waive the notice or appear at the hearing without objecting to the lack of proper notice or personal jurisdiction, the court may hear the matter immediately. A guardian of the estate or a guardian ad litem may make the waivers on behalf of the incapacitated person, and a trustee may make the waivers on behalf of any competent or incapacitated beneficiary of the trust. A consul or other representative of a foreign government, whose appearance has been entered as provided by law on behalf of any person residing in a foreign country, may make the waiver of notice on behalf of the person.
RCW 11.96A.140