RAP 9.11: ADDITIONAL EVIDENCE OF REVIEW
(a) Remedy Limited. The appellate court may direct that additional evidence on the merits of the case be taken before the decision of a case on review if: (1) additional proof of facts is needed to fairly resolve the issues on review, (2) the additional evidence would probably change the decision being reviewed, (3) it is equitable to excuse a party's failure to present the evidence to the trial court, (4) the remedy available to a party through postjudgment motions in the trial court is inadequate or unnecessarily expensive, (5) the appellate court remedy of granting a new trial is inadequate or unnecessarily expensive, and (6) it would be inequitable to decide the case solely on the evidence already taken in the trial court.
(b) Where Taken. The appellate court will ordinarily direct the trial court to take additional evidence and find the facts based on that evidence.