Wash. Rev. Code § 35.13A.050
When electing under RCW 35.13A.030 or 35.13A.040 to proceed under this section, the city may assume, by ordinance, jurisdiction of the district's responsibilities, property, facilities and equipment within the corporate limits of the city: PROVIDED, That if on the effective date of such an ordinance the territory of the district included within the city contains any facilities serving or designed to serve any portion of the district outside the corporate limits of the city or if the territory lying within the district and outside the city contains any facilities serving or designed to serve territory included within the city (which facilities are hereafter in this section called the "serving facilities"), the city or district shall for the economically useful life of any such serving facilities make available sufficient capacity therein to serve the sewage or water requirements of such territory, to the extent that such facilities were designed to serve such territory at a rate charged to the municipality being served which is reasonable to all parties.
In the event a city proceeds under this section, the district may elect upon a favorable vote of a majority of all voters within the district voting upon such propositions to require the city to assume responsibility for the operation and maintenance of the district's property, facilities and equipment throughout the entire district and to pay the city a charge for such operation and maintenance which is reasonable under all of the circumstances.
A city acquiring property, facilities and equipment under the provisions of this section shall acquire such property, facilities and equipment, and fix and collect service and other charges from owners and occupants of properties served by the city, subject, to any contractual obligations of the district which relate to the property, facilities, or equipment so acquired by the city or which are secured by taxes, assessments or revenues from the territory of the district included within the city. In such cases, the property included within the city and the owners and occupants thereof shall continue to be liable for payment of its and their proportionate share of any outstanding district indebtedness. The district and its officers shall continue to levy taxes and assessments on and to collect service and other charges from such property, or owners or occupants thereof, to enforce such collections, and to perform all other acts necessary to insure performance of the district's contractual obligations in the same manner and by the same means as if the territory of the district had not been included within the boundaries of a city.
RCW 35.13A.050