Wash. Super. Ct. Civ. R. 68

At any time more than 10 days before the trial begins, a party defending against a claim

may serve upon the adverse party an offer to allow judgment to be taken against the defending

party for the money or property or to the effect specified in the defending party’s offer, with costs

then accrued. If within 10 days after the service of the offer the adverse party serves written

notice that the offer is accepted, either party may then file the offer and notice of acceptance

together with proof of service thereof and thereupon the court shall enter judgment. An offer not

accepted shall be deemed withdrawn and evidence thereof is not admissible except in a

proceeding to determine costs. If the judgment finally obtained by the offeree is not more

favorable than the offer, the offeree must pay the costs incurred after the making of the offer. The

fact that an offer is made but not accepted does not preclude a subsequent offer. When the

liability of one party to another has been determined by verdict or order or judgment, but the

amount or extent of the liability remains to be determined by further proceedings, the party

adjudged liable may make an offer of judgment, which shall have the same effect as an offer

made before trial if it is served within a reasonable time not less than 10 days prior to the

commencement of hearings to determine the amount or extent of liability.