Wash. Rev. Code § 35.85.010
Any city of the first class shall have power to provide for the construction, maintenance and operation upon public streets and upon the extensions and connections thereof over intervening tidelands to and across any harbor reserves, waterways, canals, rivers, natural watercourses and other channels, any bridges, drawbridges, viaducts, elevated roadways and tunnels or any combination thereof together with all necessary approaches thereto, with or without street railway tracks thereon or therein, and to make any and all necessary cuts, fills, or other construction, upon, in, or along such streets and approaches as a part of any such improvement, and to order any and all work to be done which shall be necessary to complete any such improvement. The word "approaches" as used in this section shall include any arterial highway or highways or streets connecting with any such bridge, drawbridge, viaduct, elevated roadway or tunnel, or combination thereof, which are necessary to give convenient access thereto or therefrom from any portion of the improvement district which may be specially benefited by such improvement and which is liable to assessment for such improvement.
Whenever it is desired to pay the whole or any portion of the cost and expense of any such improvement by special assessments, the council or other legislative body of such city shall in the ordinance ordering such improvement fix and establish the boundaries of the improvement district, the property within which is to bear such assessment, which district shall include as near as may be, all the property specially benefited by such improvement.
RCW 35.85.010
First-class cities, generally: Chapter 35.22 RCW.