Wash. Rev. Code § 41.44.270
Should any member city of the statewide city employees retirement system established pursuant to this chapter, hereafter take advantage of any benefits lawfully available to its employees and their survivors under the old age and survivors insurance system embodied in the social security act, or should any city which has lawfully taken advantage of such benefits desire to become a member of the statewide city employees retirement system, such city and the board shall have the power to enter into an agreement mutually satisfactory to both parties adjusting the contributions to be made by such city and by its employees and the benefits to be paid by the statewide city employees retirement system, in such a manner to permit the participation of such city in the system as a member with reduced benefits to its employees and reduced contributions by the city and by its employees: PROVIDED, That such adjustment shall be made upon an actuarially sound basis and that as to all matters, other than those changed by such agreement, the provisions of the statewide city employees retirement system law shall apply: PROVIDED FURTHER, That unless such an agreement is entered into by mutual consent of such city and the board, all of the provisions of the statewide city employees retirement system law shall be applicable.
RCW 41.44.270