Wash. Rev. Code § 16.10.020
County commissioners may, if the situation so requires, establish dog control zones within high-density population districts, or other specified areas, of a county outside the corporate limits of any city, and outside the corporate limits of any organized township. For such zones, licensing regulations may be established which shall not necessarily be operative in sparsely settled rural districts, or in other portions of the county where they may not be needed. In determining the need for such zones, and in drawing their boundaries, county commissioners shall take into consideration the following factors:
If the commissioners shall find that the area proposed to be zoned is heavily populated, or that the purposes for which the land is being used therein require that dogs be controlled, or that the health, safety, and welfare of the people in the area require such control, they may propose the establishment of a dog control zone.
RCW 16.10.020