Cal. Pub. Contract Code § 7101
The state or any other public entity in any public works contract awarded to the lowest bidder, may provide for the payment of extra compensation to the contractor for the cost reduction changes in the plans and specifications for the project made pursuant to a proposal submitted by the contractor. The extra compensation to the contractor shall be 50 percent of the net savings in construction costs as determined by the public entity. For projects under the supervision of the Department of Transportation or local or regional transportation entities, the extra compensation to the contractor shall be 60 percent of the net savings, if the cost reduction changes significantly reduce or avoid traffic congestion during construction of the project, in the opinion of the public entity. The contractor may not be required to perform the changes contained in an eligible change proposal submitted in compliance with the provisions of the contract unless the proposal was accepted by the public entity.
Ca. Pub. Cont. Code § 7101