Wash. Rev. Code § 52.02.060
At the time and place of the hearing on the petition or at any adjournment thereof, the county legislative authority shall consider the petition and shall receive evidence as it deems material in favor of or opposed to the formation of the district or to the inclusion or exclusion of any lands. No lands outside of the boundaries of the proposed district as described in the petition may be included within the district without a written petition describing the land, executed by all persons having an interest of record in the lands, and filed with the proceedings on the petition. No land within the boundaries described in the petition, except that land which the county legislative authority finds will receive no benefits from the proposed district, may be excluded from the district.
RCW 52.02.060