Wash. Rev. Code § 17.21.305
The provisions of this chapter requiring all structural pest control operators, exterminators and fumigators to license with the department shall not preclude a city of the first class with a population of one hundred thousand people or more, or the county in which it is situated, from also licensing structural pest control operators, exterminators and fumigators operating within the territorial confines of said city or county: PROVIDED, That when structural pest control operators, exterminators and fumigators are licensed by both the city of the first class and the county in which the city is situated, and there exists a joint county-city health department, then the joint county-city health department may enforce the provisions of the city and county as to the license requirements for the structural pest control operators, exterminators and fumigators.
RCW 17.21.305