Iowa Code § 468.283
If the boards of the several counties acting jointly shall establish the district, they shall appoint a commission consisting of one from each county, and in addition thereto a competent engineer who shall within twenty days begin to inspect the premises and classify the lands in said district fixing the percentages and assessments of benefits and the apportionment of costs and expenses and shall complete said work within the time fixed by the boards. The qualifications of said commissioners, their classification of lands, fixing percentages and assessments of benefits and apportionment of costs and the report thereof in all details shall be governed in all respects by the provisions of subchapter I, parts 1 through 5, for districts wholly within one county.
Iowa Code § 468.283
89 Acts, ch 126, §2