Iowa Code § 384.67

Assessments levied and certified under the provisions of this subchapter, including installments and interest, are payable at the office of the county treasurer of the county where the property assessed is located, except that assessments may be paid in full or in part and without interest within thirty days after the date of certification, at the office of the county treasurer, if the property being assessed is located in an unincorporated area, or the city clerk, if the property being assessed is located in an incorporated area.