Wash. Rev. Code § 18.106.400
(1) Except as provided in this chapter, as of July 1, 2021, it is unlawful for any person, firm, partnership, corporation, or other entity to advertise, offer to do work, submit a bid, or perform any work under this chapter without being licensed as a plumbing contractor under this chapter. A plumbing contractor license expires twenty-four calendar months following the day of its issuance. An application for a plumbing contractor license must be made in writing to the department, accompanied by the required fee. The application shall state:
(d) Evidence of workers' compensation coverage for the applicant's employees working in Washington, as follows:
(5) A plumbing contractor shall:
[ 2020 c 153 s 2.]