Plumbing is a two-layer license: the business gets a plumbing contractor license, and a certified plumber has to stand behind it. Here is how both fit together.
Plumbing does not run through the general 18.27 contractor registration alone — it has its own licensing under L&I's plumbing program. To hold a plumbing contractor license you register the business, post a bond, carry insurance, and assign a designated plumber who holds a current plumber certification. No certified plumber, no license.
I work in this trade, so I will be blunt about the part people underestimate: the certification hours are real. Journey-level (PL01) is 8,000 hours — about four years — of supervised work, with a good chunk in commercial or industrial. If you are not certified yet, your realistic path to owning a plumbing company is either finishing your own hours or bringing on a certified plumber as your designated plumber. Plan for that early; it is the long pole in the tent.
Which law this comes from. Plumbing contracting is licensed under chapter 18.106 RCW, not the chapter 18.27 construction-contractor registration most trades use. The bond is $6,000 under RCW 18.106.410, and a contractor licensed under chapter 18.106 is not required to register under chapter 18.27 for the work that license covers. That is why this page does not send you to the $15,000 specialty bond, and why cleaning drains does not change the answer. If you also perform non-plumbing construction, that separate scope is where chapter 18.27 registration comes back in.
A plumber trainee can start at 16, works under direct supervision, and renews annually with eight hours of continuing education (four on code). A journey-level plumber (PL01) needs 8,000 hours — with at least 4,000 in commercial or industrial — and has to pass the journey exam. There are also specialty certifications (PL02, PL03, PL04, and others) with smaller hour requirements, roughly 2,000 to 6,000 hours depending on the specialty.
Either finish your own PL01 hours or bring on a designated certified plumber the license can stand behind.
LLC or sole prop, then Department of Revenue for your UBI.
Surety bond or an assigned savings account in that amount.
Certificate of liability at the required amount.
Notarized, with your designated plumber assigned, and pay the fees.
You cannot legally do plumbing work until the license is active.
The free builder points you to the plumbing-contractor path. The paid roadmap goes deeper on certification, bond, insurance, and designated-plumber paperwork.
Build My Free RoadmapPrimary Washington state agencies first, then supporting references. Figures verified July 2026 — always re-check current amounts before you file.