Wash. Rev. Code § 52.06.090
A part of one district may be transferred and merged with a district located within reasonable proximity if the area can be better served by the merged district. To effect such a merger, a petition, signed by a majority of the commissioners of the merging district or signed by not less than fifteen percent of the qualified electors residing in the area to be merged, shall be filed with the commissioners of the merging district, if signed by electors, or with the commissioners of the merger district if signed by commissioners of the merging district. If the commissioners of the merging district approve the petition, the petition shall be presented to the commissioners of the merger district. If the commissioners of the merger district approve the petition, an election shall be called in the area to be merged.
In the event that either board of fire district commissioners does not approve the petition, the partial merger must not proceed.
A majority of the votes cast is necessary to approve the transfer.
RCW 52.06.090