Wash. Rev. Code § 41.35.720
The required contribution rates to the retirement system for employers shall be established by the director from time to time as may be necessary upon the advice of the state actuary. The state actuary shall use the aggregate actuarial cost method to calculate contribution rates. The employer contribution rate calculated under this section shall be used only for the purpose of determining the amount of employer contributions to be deposited in the plan 2 fund from the total employer contributions collected under RCW 41.35.090.
Any increase in the contribution rate required as the result of a failure of an employer to make any contribution required by this section shall be borne in full by the employer not making the contribution.
The director shall notify all employers of any pending adjustment in the required contribution rate and such increase shall be announced at least thirty days prior to the effective date of the change.
The employer's contribution shall be remitted directly to the department within fifteen days following the end of the calendar month during which the payroll period ends.
RCW 41.35.720