Cal. Water Code § 6305

In the event the actual cost exceeds the estimated cost by more than 15 percent, a further fee shall be required by the department and shall be 115 percent of the amount by which the original fee is less than it would have been had the cost it was based upon been the same as the actual cost. No further fee shall be required, however, if that fee is to be computed at less than five hundred dollars ($500).