Wash. Rev. Code § 87.03.040
The board of county commissioners shall meet on the second Monday after the election and canvass the returns, and if it appears that at least two-thirds of all the votes cast are in favor of the district the board shall by an order declare the district duly organized and shall declare the qualified persons receiving the highest number of votes to be duly elected directors, and shall cause a certified copy of the order to be filed for record in the offices of the auditor and assessor of each county in which any portion of the district is situated. From the date of the filing the organization of the district shall be complete and the directors may, upon qualifying, enter immediately upon the duties of their office, and shall hold office until their successors are elected and qualified. Upon filing the order, the county assessor shall write the name of the district on the permanent tax roll in a column provided for that purpose opposite each description of land in the district. Such column shall be carried forward each year on the current tax roll. In the event of a change in the boundaries of a district, the assessor shall note it in the column upon the tax roll.
RCW 87.03.040