CA Pub Cont Code Section 20137

If any change or alteration of the contract is ordered, it shall be specified in writing by a duly authorized officer of the county. The cost of such change or alteration must be agreed upon between the board and the contractor unless the contract includes a provision to determine a fair and equitable price for the change or alteration. Such a provision may provide for any method of determining the price common in commercial transactions, including, but not limited to, arbitration or cost plus a fixed fee. If the cost so agreed upon:

(a)

Does not exceed the amounts specified in Sections 20121 and 21031, or

(b)

Does not exceed 10 percent of the original contract price, the board may authorize the contractor to proceed with the change or alteration without the formality of obtaining bids therefor. No change or alteration shall be authorized the amount of which is within the limitation specified in subdivision (b) and in excess of the limitation specified in subdivision (a) except by four-fifths vote of the board.