Wash. Rev. Code § 49.39.070
An employer may engage in collective bargaining with the exclusive bargaining representative and no employer may refuse to engage in collective bargaining with the exclusive bargaining representative. Upon the failure of the employer and the exclusive bargaining representative to conclude a collective bargaining agreement, any matter in dispute may be submitted by either party to the commission. If an employer implements its last and best offer where there is no contract settlement, allegations that either party is violating the terms of the implemented offer are subject to grievance arbitration procedures if and as such procedures are set forth in the implemented offer, or, if not in the implemented offer, if and as such procedures are set forth in the parties' last contract.
RCW 49.39.070