Wash. Rev. Code § 36.34.280
Whenever any county holds title to lands, for county purposes, acquired by grant, patent, or other conveyance from the United States executed under and pursuant to an act of congress, and the board of county commissioners of such county by resolution finds and determines that any portion thereof is not required for county purposes and that it would be for the best interest of the county to have such portion of the lands devoted to use by a municipality lying within the county, the board of county commissioners may, with the consent of the congress of the United States, by a proper instrument of conveyance executed by the board on behalf of the county, convey such lands to the municipality for municipal purposes, either with or without consideration, and shall not be required to advertise or offer such lands for sale or lease in the manner provided by law for the sale or lease of county property.
RCW 36.34.280