Cal. Pub. Cont. Code § 20406

If the board of supervisors is advised by the county surveyor or, in a county not having a county surveyor, by the officer having similar duties and authority, that the work can be done for a sum less than the lowest responsible bid, the board may reject all bids and order the work done by day labor, under the supervision and direction of the county surveyor or officer having similar duties and authority. In such case, the road commissioners in their respective districts shall employ all labor required, and direct the conduct of the work.