Wash. Rev. Code § 52.04.031
A petition for annexation of an area located within reasonable proximity to a fire district shall be in writing, addressed to and filed with the board of fire commissioners of the district to which annexation is desired. Such territory may be located in a county or counties other than the county or counties within which the fire protection district is located. It must be signed by the owners, according to the records of the county auditor or auditors, of not less than sixty percent of the area of land included in the annexation petition, shall set forth a legal description of the property and shall be accompanied by a plat which outlines the boundaries of the property to be annexed. The petition shall state the financial obligation, if any, to be assumed by the area to be annexed.
For the purposes of this section, "reasonable proximity" has the same meaning as in RCW 52.26.020.
RCW 52.04.031