Wash. Rev. Code § 36.40.250
In lieu of adopting an annual budget, the county legislative authority of any county may adopt an ordinance or a resolution providing for biennial budgets with a mid-biennium review and modification for the second year of the biennium. The county legislative authority may repeal such an ordinance or resolution and revert to adopting annual budgets for a period commencing after the end of a biennial budget cycle. The county legislative authority of a county with a biennial budget cycle may adopt supplemental and emergency budgets in the same manner and subject to the same conditions as the county legislative authority in a county with an annual budget cycle.
The procedure and steps for adopting a biennial budget shall conform with the procedure and steps for adopting an annual budget and with requirements established by the state auditor. The state auditor shall establish requirements for preparing and adopting the mid-biennium review and modification for the second year of the biennium.
Expenditures included in the biennial budget, mid-term modification budget, supplemental budget, or emergency budget shall constitute the appropriations for the county during the applicable period of the budget and every county official shall be limited in making expenditures or incurring liabilities to the amount of the detailed appropriation item or classes in the budget.
In lieu of adopting an annual budget or a biennial budget with a mid-biennium review for all funds, the legislative authority of any county may adopt an ordinance or a resolution providing for a biennial budget or budgets for any one or more funds of the county, with a mid-biennium review and modification for the second year of the biennium, with the other funds remaining on an annual budget. The county legislative authority may repeal such an ordinance or resolution and revert to adopting annual budgets for a period commencing after the end of the biennial budget or biennial budgets for the specific agency fund or funds. The county legislative authority of a county with a biennial budget cycle may adopt supplemental and emergency budgets in the same manner and subject to the same conditions as the county legislative authority in a county with an annual budget cycle.
The county legislative authority shall hold a public hearing on the proposed county property taxes and proposed road district property taxes prior to imposing the property tax levies.
RCW 36.40.250