Wash. Rev. Code § 35.58.500
The metropolitan municipal corporation shall have the power to levy special assessments payable over a period of not exceeding twenty years on all property within the metropolitan area specially benefited by any improvement, on the basis of special benefits conferred, to pay in whole, or in part, the damages or costs of any such improvement, and for such purpose may establish local improvement districts and enlarged local improvement districts, issue local improvement warrants and bonds to be repaid by the collection of local improvement assessments and generally to exercise with respect to any improvements which it may be authorized to construct or acquire the same powers as may now or hereafter be conferred by law upon cities. Such local improvement districts shall be created and such special assessments levied and collected and local improvement warrants and bonds issued and sold in the same manner as shall now or hereafter be provided by law for cities. The duties imposed upon the city treasurer under such acts shall be imposed upon the treasurer of the county in which such local improvement district shall be located.
A metropolitan municipal corporation may provide that special benefit assessments levied in any local improvement district may be paid into such revenue bond redemption fund or funds as may be designated by the metropolitan council to secure the payment of revenue bonds issued to provide funds to pay the cost of improvements for which such assessments were levied. If local improvement district assessments shall be levied for payment into a revenue bond fund, the local improvement district created therefor shall be designated a utility local improvement district. A metropolitan municipal corporation that creates a utility local improvement district shall conform with the laws relating to utility local improvement districts created by a city.
RCW 35.58.500
Local improvements, supplemental authority: Chapter 35.51 RCW.
Special assessments or taxation for local improvements: State Constitution Art. 7 s 9.