Wash. Rev. Code § 35.22.302
The legislative authority of every city of the first and second class owning real property, not limited by dedication or trust to a particular public use, may convey or lease for public or private use any estate, right or interest in the areas above the surface of the ground of such real property or structures or improvements thereon: PROVIDED, That the estate, right or interest so created and conveyed and the use authorized in connection therewith will not in the judgment of said legislative authority be needed for or be inconsistent with the public purposes for which such property was acquired, is being used, or to which it is to be devoted: PROVIDED FURTHER, That the legislative authority may impose conditions and restrictions on the use to be made of the estate, right or interest conveyed or leased, in the same manner and to the same extent as may be done by any vendor or lessor of real estate.
No conveyance or lease authorized by this section shall permit, authorize or suffer the lessee or grantee to encumber that portion of the real estate devoted to or needed for public purposes.
RCW 35.22.302