Iowa Code § 76.7

Counties, cities, and school corporations may at any time or times extend or renew any legal indebtedness or any part thereof they may have represented by bonds or certificates where such indebtedness is payable from a limited annual tax or from a voted annual tax, and may by resolution fund or refund the same and issue bonds therefor running not more than twenty years to be known as funding or refunding bonds, and make provision for the payment of the principal and interest thereof from the proceeds of an annual tax for the period covered by such bonds similar to the tax authorized by law or by the electors for the payment of the indebtedness so extended or renewed.