Johnson County, IA Accessory Structures: ADU Owner Occupancy (2026)
Key Facts
- Max ADUs per lot
- One per property
- Max ADU size
- 1,300 sq ft or 50% of home
- Parcels under 1 acre
- ADU must be attached
- STR owner rule
- Owner must reside on-site
- First-offense penalty
- Up to $750 civil fine
- Applies to
- Unincorporated Johnson County only
Summary
Johnson County's Unified Development Ordinance allows one auxiliary dwelling unit per property in the A, AR, SF, RS, RM, RC, and AG-T districts, capped at 1,300 square feet or half the primary home's floor area. Whenever the ADU is rented as a short-term rental, the property's legal owner must reside on site.
E. Auxiliary Dwelling Unit. Attached auxiliary dwelling units are accessory uses and detached Auxiliary dwelling units are conditionally permitted in the A, AR, SF, RS, RM, RC, and AG-T districts and are subject to the following conditions: 1. A maximum of one (1) auxiliary dwelling unit may be permitted per property. 2. The net square footage of the auxiliary dwelling unit shall not exceed one thousand three hundred (1,300) square feet of net floor area, or fifty (50) percent of the gross floor area of the primary dwelling unit, whichever is less... 5. Auxiliary dwelling units may be utilized as Short Term Rentals as allowed by this ordinance. Where an auxiliary dwelling unit is utilized as a Short Term Rental, the legal owner of the property shall reside on the property.
Official source re-checked September 7, 2026: the cited page had not changed since it was quoted.
Full Breakdown
24(E). Only one ADU is allowed per property, and its net floor area cannot exceed 1,300 square feet or 50 percent of the primary dwelling's gross floor area, whichever is smaller; garages, uncovered decks, uninhabitable attics, and low-clearance basements do not count toward that limit. Parcel size controls the ADU's form: on lots under one acre the ADU must be attached to the primary home or built into an existing accessory structure, while parcels of one acre or more may hold a standalone detached unit.
Attached ADUs must share a single electrical service with the primary home, connect through a lockable interior door, and use interconnected smoke alarms so one alarm trips every unit. 25's standard. Subsection (E)(5) adds the short-term rental trigger: an ADU may be rented short term, but only if the property's legal owner resides on the property while it is rented that way. 24(CC), which carries no owner-occupancy condition, making the ADU rule a distinct, narrower residency requirement tied specifically to ADU-based short-term rentals.
Violations & Fines
Zoning Administrator enforcement under Chapter 8:8 applies: investigating complaints, issuing county infraction citations, and withholding or revoking permits for noncompliance (subsection 8:8.2(C)). A county infraction carries a civil penalty of up to $750 for a first offense and up to $1,000 for a repeat offense of the same section by the same person (subsection 8:8.3(C)), and each day a violation continues counts as a separate offense.
Frequently Asked Questions
Does the owner-occupancy rule apply to every ADU?
Can I build a detached ADU on a half-acre lot?
What happens if I rent my ADU short term without living on the property?
Sources & Official References
Other rules in Johnson County
Compare Johnson County to another location·View the Iowa accessory structures overview
See something wrong?
Help us keep this page accurate. If you notice an error or outdated information, let us know.