Wash. Rev. Code § 50.32.020
The applicant or claimant, his or her most recent employing unit or any interested party which the commissioner by regulation prescribes, may file an appeal from any determination or redetermination with the appeal tribunal within thirty days after the date of notification or mailing, whichever is earlier, of such determination or redetermination to his or her last known address: PROVIDED, That in the event an appeal with respect to any determination is pending as of the date when a redetermination thereof is issued, such appeal, unless withdrawn, shall be treated as an appeal from such redetermination. Any appeal from a determination of denial of benefits which is effective for an indefinite period shall be deemed to be an appeal as to all weeks subsequent to the effective date of the denial for which benefits have already been denied. If no appeal is taken from any determination, or redetermination, within the time allowed by the provisions of this section for appeal therefrom, said determination, or redetermination, as the case may be, shall be conclusively deemed to be correct except as hereinbefore provided in respect to reconsideration by the commissioner of any determination.
RCW 50.32.020