After the hearing, the board of fire commissioners shall determine by resolution whether the area shall be annexed. It may annex all or any portion of the proposed area but may not include in the annexation property not described in the petition. The proposed annexation shall be subject to action by the county legislative authority, as provided under RCW 52.04.011, to the same extent as if the annexation were done under the election method of annexation. If the area proposed to be annexed under this procedure is reduced, the annexation shall occur only if the owners of not less than sixty percent of the remaining area have signed the petition. After adoption of the resolution a copy shall be filed with the county legislative authority or authorities within which the territory is located.