Wash. Rev. Code § 8.12.380
It shall be the duty of the city treasurer into whose hands such judgment and assessment roll shall come, to mail notices of such assessment to the persons whose names appear on the assessment roll, so far as the addresses of such persons are known to him or her. Any such treasurer omitting so to do, shall be liable to a penalty of five dollars for every such omission; but the validity of the special assessment shall not be affected by such omission. When any assessment or assessments are paid, it shall be the duty of the treasurer to write the word "paid" opposite the same together with the name and post office address of the person making the payment and the date of payment. The owner may annually notify the treasurer of his or her address and it shall be the duty of the treasurer to mail the notice above provided for to such address.
RCW 8.12.380