Wash. Rev. Code § 85.08.310
The said board of supervisors shall, immediately upon their election and qualification, begin the construction of such system of improvement and shall proceed with the construction thereof in accordance with the plans adopted therefor. In the construction of any system of drainage, construction shall be begun at the outlet or outlets thereof and at such other points as may be deemed advisable from time to time. In the construction of any system of improvement the board of supervisors with the approval of the board of county commissioners may modify, curtail, enlarge or add to the original plans wherever the same may be found necessary or advisable in the course of actual construction. But such changes shall not in the aggregate increase the estimated cost of the entire system by more than one-fifth, and all additional or different rights-of-way required shall be obtained as hereinbefore prescribed. The board of county commissioners may in its discretion let the construction of said system or any portion thereof by contract, in the manner provided for letting contracts for the construction of county roads and bridges. The board of county commissioners may, upon such terms as may be agreed upon by the United States acting in pursuance of the National Reclamation Act approved June 17, 1902 (32 Statutes at Large 388), and the acts amendatory thereof and supplemental thereto, or in pursuance to any other act of congress appropriate to the purpose, contract for the construction of the system of improvement or any part thereof, by the United States, or in cooperation with the United States therein. In such case, no bond shall be required, and the work shall be done under the supervision and control of the proper officers of the United States.
Unless the work of construction is let by contract as hereinbefore provided, or for such part of such work as is not covered by contract, the board of supervisors shall employ such number of persons as shall be necessary to successfully carry on the work of such construction, and shall give preference in such employment to persons owning land to be benefited by the improvement.
The provisions of this section shall not be construed as denying to the supervisors, in case the construction work is left in their hands, the power to enter into an agreement with any contractor to furnish labor, material, equipment and skilled supervision, the contractor to be compensated upon the basis of a specific sum, or upon a percentage of the cost of the work, the services of the contractor to cover the use of equipment and the value of skilled supervision: PROVIDED, HOWEVER, That there is retained in the said board by the contract the right of termination thereof at any time, on reasonable notice, and fixing in the said contract, or reserving in said board, the right to fix the rates of wages to be paid to the persons employed in said work. The board of supervisors may also let contracts in such manner and on such notice as they deem advisable for items of construction not exceeding one thousand dollars in amount of expenditures.
RCW 85.08.310