Wash. Rev. Code § 35A.14.400
If a portion of a fire protection district including less than sixty percent of the assessed value of the real property of the district is annexed to or incorporated into a code city, the ownership of all assets of the district shall remain in the district and the district shall pay to the code city within one year or within such period of time as the district continues to collect taxes in such incorporated or annexed areas, in cash, properties or contracts for fire protection services, a percentage of the value of said assets equal to the percentage of the value of the real property in the entire district lying within the area so incorporated or annexed: PROVIDED, That if less than five percent of the area of the district is affected, no payment shall be made to the code city except as provided in RCW 35.02.205. The fire protection district shall provide fire protection to the incorporated or annexed area for such period as the district continues to collect taxes levied in such annexed or incorporated area.
RCW 35A.14.400