Wash. Rev. Code § 50.20.180
If waiting period credit or the payment of benefits shall be denied to any claimant for any week or weeks, the claimant and such other interested party as the commissioner by regulation prescribes shall be promptly issued written notice of the denial and the reasons therefor. In any case where the department is notified in accordance with such regulation as the commissioner prescribes or has reason to believe that the claimant's right to waiting period credit or benefits is in issue because of his or her separation from work for any reason other than lack of work, the department shall promptly issue a determination of allowance or denial of waiting period credit or benefits and the reasons therefor to the claimant, his or her most recent employing unit as stated by the claimant, and such other interested party as the commissioner by regulation prescribes. Notice that waiting period credit or benefits are allowed or denied shall suffice for the particular weeks stated in the notice or until the condition upon which the allowance or denial was based has been changed.
RCW 50.20.180