RCW 35.43.190: Work—By contract or by city or public corporation.

All local improvements, the funds for the making of which are derived in whole or in part from assessments upon property specially benefited shall be made by contract on competitive bids whenever the estimated cost of such improvement including the cost of materials, supplies, labor, and equipment will exceed the sum of five thousand dollars. The city, town, or public corporation may reject any and all bids. The city, town, or public corporation itself may make the local improvements if all the bids received exceed by ten percent preliminary cost estimates prepared by an independent consulting engineer or registered professional engineer retained for that purpose by the city, town, or public corporation.