Wash. Rev. Code § 35.13.178
The legislative body of the city or town shall hold two or more public hearings, to be held at least thirty days apart, upon the proposed comprehensive plan, giving notice of the time and place thereof by publication in a newspaper of general circulation in the annexing city or town and the area to be annexed. A copy of the ordinance or resolution adopting or embodying such proposed plan or any part thereof or any amendment thereto, duly certified as a true copy by the clerk of the annexing city or town, shall be filed with the county auditor. A like certified copy of any map or plat referred to or adopted by the ordinance or resolution shall likewise be filed with the county auditor. The auditor shall record the ordinance or resolution and keep on file the map or plat.
RCW 35.13.178