Wash. Rev. Code § 70.44.210
As an alternate method of annexation to public hospital districts, any territory adjacent to a public hospital district may be annexed thereto by vote of the qualified electors residing in the territory to be annexed, in the manner provided in RCW 70.44.210 through 70.44.230. An election to annex such territory may be called pursuant to a resolution calling for such an election adopted by the district commissioners.
Any resolution calling for such an election shall describe the boundaries of the territory to be annexed, state that the annexation of such territory to the public hospital district will be conducive to the welfare and benefit of the persons or property within the district and within the territory proposed to be annexed, and fix the date, time and place for a public hearing thereon which date shall be not more than sixty nor less than forty days following the adoption of such resolution.
RCW 70.44.210