Wash. Rev. Code § 91.08.530
After the confirmation of the assessment roll of any improvement district provided for herein, the board shall proceed at once with the construction of the improvement, and in carrying on the construction it shall have full charge and management thereof and the power to employ such assistants as it may deem necessary, and purchase all material required in such construction; and it shall have power to let the whole or any part of the work of the improvement to the lowest and best bidder therefor, after public advertisement and call for bids; and in case of such letting of a contract it shall have the power also to enter into all necessary agreements with the contractor in the premises: PROVIDED, That in the case of the letting of a contract the board shall require the contractor to give a bond in the amount of the contract price, with sureties to be approved by the board and running to the board as obligee therein, conditioned for the faithful and accurate performance of his or her contract by the contractor, and that he or she will pay, or cause to be paid, all just claims of all persons performing labor upon or rendering services in doing the work, or furnishing materials, merchandise or provisions used by the contractor in the construction of the improvement. The bond shall be filed and recorded in the office of the auditor of the county and every subcontractor on any such work shall file and record a like bond in the full amount of his or her subcontract. Unless otherwise paid their claims for labor or services, materials, merchandise or provisions, the claimants may have recourse by suit upon such bond in their own names: PROVIDED, That no such claim or suit shall be maintained unless the persons making the claim shall within thirty days after the completion of the improvement, file their claims, duly verified, to the effect that the amounts thereof are just and due and are unpaid, with the clerk of the board. Each bidder for a contract to be let under this section shall deliver with his or her bid a check for five percent of the amount of the bid, drawn upon a bank in this state and certified by the bank, as surety to the board that the bidder will enter into the contract with the board. The checks of unsuccessful bidders will be returned to them when an award of the contract has been made by the board. A low bidder who claims error and fails to enter into a contract is prohibited from bidding on the same project if a second or subsequent call for bids is made for the project.
RCW 91.08.530
Contractor's bond: Chapter 39.08 RCW.