Iowa Code § 468.56: Payment of assessments.

All assessments for benefits, as corrected and approved by the board, shall be levied at one time against the property benefited, and when levied and certified by the board, are payable at the office of the county treasurer. A person may pay the person’s assessment in full without interest within thirty days after the levy of assessments, and before any improvement certificates or drainage bonds are issued for the assessment, and may pay a certificate at any time after issue, with accrued interest.