Wash. Rev. Code § 35.02.225
It is the desire of the legislature that the citizens of newly incorporated cities or towns receive uninterrupted and adequate services in the period prior to the city or town government attaining the ability to provide such service levels. In addition to the services provided under RCW 35.02.220, it is the purpose of this section to permit the county or counties in which a newly incorporated city or town is located to contract with the newly incorporated city or town for the continuation of essential services until the newly incorporated city or town has attained the ability to provide such services at least at the levels provided by the county before the incorporation. These essential services may include but are not limited to, law enforcement, road and street maintenance, drainage, and other utility services previously provided by the county before incorporation. The contract should be negotiated on the basis of the county's cost to provide services without consideration of capital assets which do not continue to be amortized for principal and interest or depreciated by the county. The exception for not considering capital assets which are no longer amortized for principal and interest or depreciated is recognition of the preexisting financial investment of citizens of the newly incorporated city or town have made in county capital assets.
Nothing in this section limits the ability of the county and the newly incorporated city or town to contract for higher service levels or for other time periods than those imposed by this section.
RCW 35.02.225