Wash. Super. Ct. Civ. Arb. R. SCCAR 5.4
The arbitration hearing may proceed, and an award may be made, in the absence of any party who after due notice fails to participate or to obtain a continuance. If a defendant is absent, the arbitrator shall require the plaintiff to submit the evidence required for the making of an award. In a case involving more than one defendant, the absence of a defendant does not preclude the arbitrator from assessing as part of the award damages against the defendant or defendants who are absent. The arbitrator, for good cause shown, may allow an absent party an opportunity to appear at a subsequent hearing before making an award. A party who fails to participate without good cause waives the right to a trial de novo.
Wash. Super. Ct. Civ. Arb. R. SCCAR 5.4