Wash. Rev. Code § 36.35.170
The county or its successors in interest or assigns shall have authority to include in one action any and all tracts of land in which plaintiff or plaintiffs in such action, jointly or severally, has or claims to have an interest. Such action shall be one in rem as against every right and interest in and claim against any and every part of the real property involved, except so much thereof as may be at the time the summons and notice is filed with the clerk of the superior court in the actual, open and notorious possession of any person or corporation, and then except only as to the interest claimed by such person so in possession: PROVIDED, That the possession required under the provisions of RCW 36.35.160 through 36.35.270 shall be construed to be that by personal occupancy only, and not merely by representation or in contemplation of law. No person, firm or corporation claiming an interest in or to such lands need be specifically named in the summons and notice, except as in RCW 36.35.160 through 36.35.270, and no pleadings other than the summons and notice and the written statements of those claiming a right, title and interest in and to the property involved shall be required.
RCW 36.35.170