No municipality shall order any improvement to be paid for by local assessment where the estimated costs of such improvement, if such costs are to be assessed to the property in the district, or that portion of the estimated costs to be assessed, if a portion only of said total costs are to be assessed, when added to all other outstanding and unpaid local improvement assessments against the property included in the district, excluding penalties and interest, shall exceed the actual value of the real property, including the value of the improvements thereon.
The council shall provide, by ordinance, the method of determining the actual value of the real property including the improvements thereon in the district and when the valuation is so determined, such valuation shall be final and conclusive in the absence of fraud or gross mistake.