Wash. Rev. Code § 4.44.210
The jurors having been examined as to their qualifications, first by the plaintiff and then by the defendant, and passed for cause, the peremptory challenges shall be conducted as follows, to wit:
The plaintiff may challenge one, and then the defendant may challenge one, and so alternately until the peremptory challenges shall be exhausted. During this alternating process, if one of the parties declines to exercise a peremptory challenge, then that party may no longer peremptorily challenge any of the jurors in the group for which challenges are then being considered and may only peremptorily challenge any jurors later added to that group. A refusal to challenge by either party in the said order of alternation shall not prevent the adverse party from using the full number of challenges.
RCW 4.44.210