Cal. Pub. Cont. Code § 20452

(a) No changes, except as provided in Section 20453, shall be made pursuant to this chapter that will increase the estimated assessable cost by more than 20 percent of the total estimated cost of the work as determined from either of the following:
1. (1) The engineer’s estimate, if the change is ordered prior to the award of the contract.
2. (2) The successful bid, if the change is ordered after the award of the contract.
(b) Any changes made pursuant to subdivision (a) shall also be subject to the limitations, if any, contained in any law applicable to the proceedings that imposes limitations upon the amount by which the estimated cost of the work or improvement may be increased by reason of those changes.