RCW 51.52.112

All taxes, penalties, and interest shall be paid in full before any action may be instituted in any court to contest all or any part of such taxes, penalties, or interest unless the court determines that there would be an undue hardship to the employer. In the event an employer prevails in a court action, the employer shall be allowed interest on all taxes, penalties, and interest paid by the employer but determined by a final order of the court to not be due, from the date such taxes, penalties, and interest were paid. Interest shall be at the rate allowed by law as prejudgment interest.