Wash. Rev. Code § 52.02.070
The county legislative authority has the authority to consider the petition and, if it finds that the lands or any portion of the lands described in the petition, and any lands added thereto by petition of those interested, will be benefited and that the formation of the district will be conducive to the public safety, welfare, and convenience, it shall make a finding by resolution; otherwise it shall deny the petition. The county legislative authority shall consider only those areas located within the county when considering the petition. If the county legislative authority approves the petition, it shall designate the name and number of the district, fix the boundaries of the district that are located within the county, and direct that an election be held within the proposed district for the purpose of determining whether the district shall be organized under this title and for the purpose of the election of its first fire commissioners.
Where a proposed fire protection district is located in more than a single county, the fire protection district shall be identified by the name of each county in which the proposed fire protection district is located, listed alphabetically, followed by a number that is the next highest number available for a fire protection district in the one of these counties that has the greatest number of fire protection districts. An election on a proposed fire protection district that is located in more than one county shall not be held unless the proposed district has been approved by the county legislative authorities, or boundary review boards, of each county within which the proposed district is located.
RCW 52.02.070