RCW 18.27.010 (1)
(1)(a) “Contractor” includes any person, firm, corporation, or other entity who or which, in the pursuit of an independent business undertakes to, or offers to undertake, or submits a bid to, construct, alter, repair, add to, subtract from, improve, develop, move, wreck, or demolish any building, highway, road, railroad, excavation or other structure, project, development, or improvement attached to real estate or to do any part thereof including the installation of carpeting or other floor covering, the erection of scaffolding or other structures or works in connection therewith, the installation or repair of roofing or siding, performing tree removal services, or cabinet or similar installation; or, who, to do similar work upon his or her own property, employs members of more than one trade upon a single job or project or under a single building permit except as otherwise provided in this chapter.
(b) “Contractor” also includes a consultant acting as a general contractor.
(c) “Contractor” also includes any person, firm, corporation, or other entity covered by this subsection (1), whether or not registered as required under this chapter or who are otherwise required to be registered or licensed by law, who offer to sell their property without occupying or using the structures, projects, developments, or improvements for more than one year from the date the structure, project, development, or improvement was substantially completed or abandoned. A person, firm, corporation, or other entity is not a contractor under this subsection (1)(c) if the person, firm, corporation, or other entity contracts with a registered general contractor and does not superintend the work.

WAC 296-200A-110
Before issuing a building permit, a city, county, or town must verify the registration of the general or specialty contractor applying for the permit.