Streets and Highways Code § 977
CHAPTER 2. Powers and Duties of Boards of Supervisors
The board of supervisors of any county desiring to act under section 976 may, by unanimous vote of its membership, determine what real property is necessary and proper for the uses and purposes of county highways. Any county thus desiring to purchase or lease with the consent of the owner, or to obtain, hold, improve or maintain, real property or an interest therein situated within an adjoining county shall first obtain the consent by resolution of the board of supervisors of the adjoining county.