Cal. Sts. & High. Code § 2901

If the total cost of any improvement, including incidental expenses but excluding the actual cost of the acquisition of any property, exceeds the total estimated cost including incidental expenses, as stated in the report and the separate estimate for changes, if any, in the things to be done as filed with the clerk, by more than one-tenth of such estimated cost, no part of the excess over said one-tenth shall be assessed upon the lands to be assessed in the proceeding.