Wash. Rev. Code § 51.48.210
If payment of any tax due is not received by the department by the due date, there shall be assessed a penalty of five percent of the amount of the tax for the first month or part thereof of delinquency; there shall be assessed a total penalty of ten percent of the amount of the tax for the second month or part thereof of delinquency; and there shall be assessed a total penalty of twenty percent of the amount of the tax for the third month or part thereof of delinquency. No penalty so added may be less than ten dollars. If a warrant is issued by the department for the collection of taxes, increases, and penalties, there shall be added thereto a penalty of five percent of the amount of the tax, but not less than five dollars nor greater than one hundred dollars. In addition, delinquent taxes shall bear interest at the rate of one percent of the delinquent amount per month or fraction thereof from and after the due date until payment, increases, and penalties are received by the department.
RCW 51.48.210
Conflict with federal requirements-Severability-Effective date-1987 c 111: See notes following RCW 50.12.220.