After the requirements of this chapter have been satisfied and a contract and bond for any work for which a special assessment is to be levied have been approved by the district board, the board may direct special assessments to be levied for the payment of appropriate costs and the secretary shall certify to the board the items of total cost to be paid by special assessments so far as they have been ascertained. The certificate must include the estimated construction cost under the terms of any contract, a reasonable allowance for cost of extra work that may be authorized under the plans and specifications, acquisition of right of way, engineering, fiscal agents’ and attorney’s fees for any services in connection with the authorization and financing of the improvement, cost of publication of required notices, printing of improvement bonds, cost necessarily paid for damages caused by such improvement, interest during the construction period, and all expenses incurred in making the improvement and levy of assessments.

A contract or contracts may not be awarded which exceed, by forty percent or more, the estimated cost of the project as presented to and approved by the affected landowners.