Cal. Gov. Code § 25549.5

The board of supervisors may lease to any private person, firm, or corporation, any real property which belongs to the county or city and county if the instrument by which such property is let requires the lessee therein to construct on the demised premises, or provide for the construction thereon, of a building or buildings, for the joint use of the county or city and county and the private person, firm, or corporation during the term of the lease or agreement. Title to such portion of the building to be occupied by the county or city and county may vest in the county or city and county.