Wash. Rev. Code § 51.48.120
If any employer should default in any payment due to the state fund the director or the director's designee may issue a notice of assessment certifying the amount due, which notice shall be served upon the employer by mailing such notice to the employer by a method for which receipt can be confirmed or tracked to the employer's last known address or served in the manner prescribed for the service of a summons in a civil action. Such notice shall contain the information that an appeal must be filed with the board of industrial insurance appeals and the director by mail or personally within thirty days of the date of service of the notice of assessment in order to appeal the assessment unless a written request for reconsideration is filed with the department of labor and industries.
RCW 51.48.120