Wash. Rev. Code § 35.96.030
Every city or town shall have the power to convert existing overhead electric and communication facilities to underground facilities pursuant to RCW 35.43.190 where such facilities are owned or operated by the city or town. Where such facilities are not so owned or operated, every city or town shall have the power to contract with electric and communication utilities, as hereinafter provided, for the conversion of existing overhead electric and communication facilities to underground facilities. To provide funds to pay the whole or any part of the cost of any such conversion, either where the existing overhead electric and communication facilities are owned or operated by the city or town or where they are not so owned or operated, every city or town shall have the power to create local improvement districts and to levy and collect special assessments against the real property specially benefited by such conversion. For the purpose of ascertaining the amount to be assessed against each lot or parcel of land within any local improvement district established pursuant to this chapter, in addition to other methods provided by law for apportioning special benefits, the legislative authority of any city or town may apportion all or part of the special benefits accruing on a square footage basis or on a per lot basis.
RCW 35.96.030