Wash. Rev. Code § 17.28.340
If upon the hearing the district board finds that the petition and the proceedings thereon meet the requirements of this chapter and that it is desirable and to the interests of the district and of the territory proposed to be annexed that the territory, with boundaries as fixed and determined by the district board, or any portion of it, should be annexed to the district, the board shall order the boundaries of the district changed to include the territory, or portion of the territory, subject to approval of the electors of the territory proposed to be annexed. The election to be conducted and the returns canvassed and declared insofar as is practicable in accordance with the requirements of this chapter for the formation of a district. The expenses of such election shall be borne by the mosquito control district regardless of the outcome of the election.
The order of annexation shall describe the boundaries of the annexed territory and that portion of the boundary of the district which coincides with any boundary of the territory. If necessary in making this order, the board may have any portion of the boundaries surveyed.
If more than one petition for the annexation of the territory has been presented, the district board may in one order include in the district any number of separate territories.
RCW 17.28.340