Washington is one of four monopolistic states — workers' comp is administered through the state Department of Labor & Industries (L&I) rather than private insurers, with narrow exceptions for self-insured employers.
You register with L&I when you hire your first employee. Rates are set by risk class — a roofer pays roughly 10x what an office admin pays because injury frequency and severity differ that much.
Owners and officers can usually elect out of coverage on themselves (with the right paperwork), but if you elect out and get hurt, there's no L&I safety net — you'd rely on personal disability and health insurance instead.
The audit at year-end is where new employers get surprised. L&I bases premium on actual hours worked in each class, not what you estimated. Under-reporting means back-premium plus penalties.
If your business has employees in other states, you need a separate voluntary workers' comp policy for that state — L&I only covers Washington work. Multi-state contractors and remote employers hit this constantly.