Wash. Rev. Code § 35A.14.310
A code city may annex an unincorporated area contiguous to the city that is owned by the federal government by adopting an ordinance providing for the annexation and which ordinance either acknowledges an agreement of the annexation by the government of the United States, or accepts a gift, grant, or lease from the government of the United States of the right to occupy, control, improve it or sublet it for commercial, manufacturing, or industrial purposes: PROVIDED, That this right of annexation shall not apply to any territory more than four miles from the corporate limits existing before such annexation. Whenever a code city proposes to annex territory under this section, the city shall provide written notice of the proposed annexation to the legislative authority of the county within which such territory is located. The notice shall be provided at least thirty days before the city proposes to adopt the annexation ordinance. The city shall not adopt the annexation ordinance, and the annexation shall not occur under this section, if within twenty-five days of receipt of the notice, the county legislative authority adopts a resolution opposing the annexation, which resolution makes a finding that the proposed annexation will have an adverse fiscal impact on the county or road district.
RCW 35A.14.310