Iowa Code § 468.393: Costs assessed.

If said district is established, the entire costs and expenses incurred under this part shall be assessed against and collected from the lands lying within such district, by the levy of a rate upon the assessable value of the land and improvements within such district, sufficient to raise the required sum; provided the board may, in their discretion, classify the land within such district and graduate the tax thereon, as provided in subchapter I.