Wash. Rev. Code § 86.09.271
The office of the directors and principal place of business of the district shall be located, if possible, at some place within the district to be designated by the board. If a place convenient and suitable for conducting district business and public hearings required by this chapter cannot be found within the district, the office may be located in the county within which the major portion of district lands is situated. The office and place of business cannot thereafter be changed, except with the previous written consent of the county legislative authority of the county within which the major portion of the district is situated, and without passing a resolution to that effect at a previous regular meeting of the board, entered in the minutes thereof and without posting a notice of the change in a conspicuous public place at or near the place of business which is to be changed at least ten days prior thereto and by the previous posting of a copy of the notice for the same length of time at or near the new location of the office.
RCW 86.09.271