Wash. Rev. Code § 70A.100.140
For any new or revised water or sewer system plan submitted for review under this chapter, the department of health shall review and either approve, conditionally approve, reject, or request amendments within ninety days of the receipt of the submission of the plan. The department of health may extend this ninety-day time limitation for new submittals by up to an additional ninety days if insufficient time exists to adequately review the general comprehensive plan. For rejections of plans or extensions of the timeline, the department shall provide in writing, to the person or entity submitting the plan, the reason for such action. In addition, the person or entity submitting the plan and the department of health may mutually agree to an extension of the deadlines contained in this section.
RCW 70A.100.140