Wash. Rev. Code § 35.85.050
Any city of the first class shall have power to provide for the construction, maintenance and operation within such city of tunnels, subways, or both, with or without roadways, sidewalks, street railway tracks or any combination thereof therein, together with all necessary approaches thereto; 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 tunnel or subway which may be 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 in which is to bear such assessment, which district shall include as near as may be all the property specially benefited by such improvement.
RCW 35.85.050