Iowa Code § 468.99
Whenever any special assessment upon any lands within any drainage district shall have been adjudged to be void for any jurisdictional defect or for any illegality or uncertainty as to the terms of any contract and the improvement shall have been wholly completed, the board or boards of supervisors shall have power to remedy such illegality or uncertainty as to the terms of any such contract with the consent of the person with whom such contract shall have been entered into and make certain the terms of such contract and shall then cause a reassessment of such land to be made on an equitable basis with the other land in the district by taking the steps required by law in the making of an original assessment and relevying the tax in accordance with such assessment, and such tax shall have the same force and effect as though the board or boards of supervisors had jurisdiction in the first instance and no illegality or uncertainty existed in the contract.
Iowa Code § 468.99
89 Acts, ch 126, §2