ADU rules in Marshalltown

Detached ADUs are permitted by right as an accessory use to a single-family detached house in the AG, RR, RL, RM, RH and MU districts, and to a single-family attached dwelling in RL, RM, RH, MU and UC. Ord. 15104 (adopted 6-23-2025, ahead of the 7-1-2025 state law) removed the old 15,000 sq ft minimum lot size and the special use permit that RM and RH used to require. An ADU may sit in a detached garage or other accessory building, but not in a structure without a permanent foundation; a manufactured or mobile home used as an ADU must be placed on a permanent foundation and assessed as real property.

An 'integrated ADU' (created inside the house or by adding floor area to it) is allowed on the same terms as a detached one, by right, in every district where a single-family house or single-family attached dwelling is permitted. The state floor in Iowa Code §364.3(23)(e) covers ADUs attached to or detached from the house.

Since Ord. 15104 the city's cap matches the state floor: an ADU may be up to 1,000 sq ft or half the size of the single-family residence, whichever is larger, measured inside the ADU only (a garage or workshop in the same building is not counted). The ADU follows the same setbacks as the house in its district. The city has not yet written in the 2026 amendment (SF 2369, effective 7-1-2026), under which the ADU's size excludes unfinished basements and the house's 'size' is its gross living area excluding garages, decks and unheated porches; those state definitions apply anyway. The generic accessory-structure limits in §156.C.007(C) and Table 156.C.007-1 (for example an 18 ft general cap and 15 ft on lots of half an acre or less) cannot lawfully be applied to a detached ADU more strictly than the house's own limits.

The current code has no owner-occupancy rule: Ord. 15104 deleted the 2022 requirement that the owner live in the house or the ADU when a detached ADU was built in RL, RM, RH or MU. State law separately bars any requirement that the property owner be a resident. The general accessory-use rule that an accessory use stay under the same ownership as the principal use still applies, which keeps the ADU from being sold off separately.

Table 156.E.004 lists no minimum or maximum parking for an 'Accessory Dwelling Unit' (both columns '--'), so only the house's own 2 spaces per dwelling unit apply. Ord. 15104 removed the former rule requiring 1 extra off-street space for an ADU in RL, RM and RH (and the 1-per-unit table entry). State law bars added parking or an in-lieu fee.

'Dwelling, Accessory' is a P use, 'permitted by right', in every district that allows it, so an ADU needs only the ordinary zoning and building review; single-family construction does not need a site plan. Before Ord. 15104 ADUs were S (special use, public hearing) in RM and RH. State law requires approval without discretionary review or hearing on the single-family timeline, with any denial in writing.

An ADU may be rented. Every renter-occupied dwelling unit needs an application and fee before occupancy, a certificate of compliance after passing inspection, and yearly registration under the Housing Code; units occupied by the owner or the owner's immediate family are not 'renter-occupied'. The zoning code lists 'short-term rental or homestay' as a typical accessory use to household living, but its definitions limit a homestay to 4 guests and 90 days a year inside the principal building, and say dwelling units are for stays of 30 days or more unless otherwise specified. Under Iowa Code 414.1(1)(e) a city cannot regulate, restrict or license short-term rentals beyond general safety, nuisance and similar rules enforced the same way as for other homes, so those short-term-specific limits are unenforceable.