Wash. Rev. Code § 51.04.120
Any employer other than a self-insurer subject to this title shall, under such rules as the department shall prescribe, apply for and obtain from the department a certificate of coverage. The certificate shall be personal and nontransferable and shall be valid as long as the employer continues in business and pays the taxes due the state. In case the employer maintains more than one place of business, a separate certificate of coverage for each place at which business is transacted shall be required. Each certificate shall be numbered and shall show the name, residence, and place and character of business of the employer and such other information as the department deems necessary and shall be posted conspicuously at the place of business for which it is issued. Where a place of business of the employer is changed, the employer must notify the department within thirty days of the new address and a new certificate shall be issued for the new place of business. No employer may engage in any business for which taxes are due under this title without having a certificate of coverage in compliance with this section, except that the department, by general rule, may provide for the issuance of a certificate of coverage to employers with temporary places of business.
RCW 51.04.120
Engaging in business without certificate of coverage-Unlawful actions-Penalties: RCW 51.48.103.