Cal. Pub. Cont. Code § 20416

Whenever the total amount bid by the lowest responsible bidder plus the estimated amount of incidental costs would result in assessments under this division which exceed by more than 15 percent the engineer’s estimate of cost of the proposed work to be assessed, as stated in the “Notice of Improvement” referred to in Section 5193 of the Streets and Highways Code, the legislative body shall direct the clerk to give notice by mail of such increase over the estimated cost.