Electric skateboards are not regulated in the state of Washington, however individual cities and counties may create specific regulations.
By City
Seattle
Skateboards (motors or not) are considered “toys” in the City of Seattle (Seattle Municipal Code 11.40.250) and therefore are not allowed on streets.
Electric personal assistive mobility devices (EPAMDs) are regulated by SMC 11.14.186, but only include devices with two wheels or are self balancing. Therefore, electric skateboards are not regulated as EPAMDs, but OneWheel, SoloWheel, and electric scooters are. EPAMDs are permitted on roadways (if the road speed limit is less than 35 mph and it is not an arterial with a sidewalk), shoulders, and sidewalks but not on bike paths (SMC 11.46.010).
Legal disclaimer: I am not a lawyer, and this article is not to be construed as legal advice. Use common sense, and always check your local laws and ordinances to ensure you are in compliance. Neither E-Skate Central nor its authors are liable for incomplete or incorrect information.