Wash. Rev. Code § 36.88.430
Every county shall have the power to contract with electric and communication utilities, as hereinafter provided, for any or all of the following purposes:
To provide funds to pay the whole or any part of the cost of any such conversion or initial installation, together with the expense of furnishing electric energy, maintenance and operation to any ornamental street lighting facilities served from underground electrical facilities, every county shall have the power to create county road improvement districts and to levy and collect special assessments against the real property specially benefited by such conversion or initial installation. For the purpose of ascertaining the amount to be assessed against each lot or parcel of land within any county road improvement district established pursuant to RCW 36.88.410 through 36.88.480, in addition to other methods provided by law for apportioning special benefits, the county commissioners may apportion all or part of the special benefits accruing on a square footage basis or on a per lot basis.
That portion of the assessments levied in any county road improvement district to pay part of the cost of the initial installation of underground electric and communication facilities shall not exceed the cost of such installation, less the estimated cost of constructing overhead facilities providing equivalent service.
RCW 36.88.430