Iowa Code § 357.29: Subdistricts.

1. If the cost of the desired extensions will be as much as five thousand dollars, the interested parties may petition the board of supervisors to organize a subdistrict, and in such case the board shall proceed in the same manner as for a new district, and may take in territory not originally assessed.

2. The board of supervisors shall have power at any time to alter the boundaries of any district prior to the time of posting or publishing notice of the election within the district.