Wash. Rev. Code § 85.38.170
Budgets for each special district shall be adopted, and special assessments imposed, annually for the succeeding calendar year. On or before December 1st of each year, the governing body of the special district shall adopt a resolution approving a budget for the succeeding year and special assessments sufficient to finance the budget. A copy of the resolution and the budget shall be forwarded immediately to the county legislative authority of the county or counties within which the special district is located and to the treasurer of the county or counties in which the special district is located. Special assessments necessary to generate funds for this budget shall be imposed pursuant to the system or systems of assessment established by the county. Special assessments shall be collected by the county treasurer or treasurers within which the special district is located. Notice of the special assessments due may be included on the notice of property taxes due, may be included on a separate notice that is mailed with the notice of property taxes due, or may be sent separately from the notice of property taxes due. Special assessments shall be due at the same time property taxes are due and shall constitute liens on the land or improvements upon which they are imposed. Delinquent special assessments shall be foreclosed in the same manner, and subject to the same time schedules, interest, and penalties as delinquent property taxes. County treasurers may impose a fee for collecting special assessments not to exceed one percent of the dollar value of special assessments collected.
RCW 85.38.170