Wash. Rev. Code § 35.61.220
If at any time any proposed improvement of any parkway, avenue, street, or boulevard is deemed by the board of metropolitan park commissioners to be a special benefit to the lands adjoining, contiguous, approximate to or in the neighborhood of the proposed improvement, which lie within the city, the board may so declare, describing the property to be benefited. Thereupon they may petition the city council to cause the improvement contemplated by the commissioners to be done and made on the local assessment plan, and the portion of the cost of the improvement as fixed by such assessment roll to be assessed against the said property so benefited in the same manner and under the same procedure as of other local improvements, and the remainder of the cost of such improvement to be paid out of the metropolitan park district fund.
The board of park commissioners shall designate the kind, manner and style of the improvement to be made, and may designate the time within which it shall be made.
RCW 35.61.220
Local improvements, supplemental authority: Chapter 35.51 RCW.