Johnson County, IA Accessory Structures: ADU Rules (2026)
ADU rules in Johnson County, IA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- Max ADUs per property
- One
- Size cap
- 1,300 sq ft or 50% of primary home
- Detached ADU
- Requires 1+ acre parcel and CUP
- Attached ADU districts
- A, AR, SF, RS, RM, RC, AG-T
- Mobile units
- Not allowed as ADUs
Summary
Johnson County treats attached auxiliary dwelling units as an accessory use and detached units as a conditional use in the A, AR, SF, RS, RM, RC, and AG-T districts under UDO Section 8:1.24(E). The county limits each property to one ADU capped at 1,300 square feet or half the primary home's floor area, whichever is smaller, and restricts detached units to parcels of one acre or more.
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: ... 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. ... 4. Parcel Size. Auxiliary dwelling units on parcels less than one (1) acre shall be attached to the primary dwelling unit, or be attached to or integrated within an existing accessory structure. Auxiliary dwelling units on parcels one (1) acre or larger may be built as a standalone secondary dwelling.
Official source re-checked September 7, 2026: the cited page had not changed since it was quoted.
Full Breakdown
29, in the A, AR, SF, RS, RM, RC, and AG-T districts. Only one auxiliary dwelling unit 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 less; net floor area calculations exclude garages, uncovered decks, uninhabitable attics, and below-grade storage under six feet eight inches of ceiling height. The unit must meet the primary dwelling's setback requirements and cannot cause the zone's lot coverage requirement to be exceeded.
Parcel size controls the ADU's form: on parcels under one acre, the ADU must be attached to the primary dwelling or integrated into an existing accessory structure, while parcels of one acre or larger may build a standalone detached unit. 24(E)(6) requires the two units to share a lockable interconnecting door, a single electrical utility service, and interconnected smoke alarms so that one unit's alarm triggers both. 25 parking table. These conditions were revised by Ordinance 10-10-24-01, the same October 2024 amendment that adjusted ADU density calculations and added cryptocurrency mining as an allowed MH-district use, and by Ordinance 09-26-24-01, which updated the 1,300-square-foot cap and the setback and lot-coverage cross-reference.
Violations & Fines
Building a detached ADU without the required Conditional Use Permit, exceeding the 1,300-square-foot or 50 percent floor-area cap, or placing a standalone unit on a parcel under one acre is a County Infraction under Section 8:8.3, carrying a civil penalty up to $750 for a first offense and up to $1,000 for a repeat violation, with criminal penalties up to $625 and 30 days in jail available under Section 8:8.4.
Frequently Asked Questions
How big can an auxiliary dwelling unit be in Johnson County?
Can I build a detached ADU on a small lot?
What safety features does an attached ADU need?
Sources & Official References
Other rules in Johnson County
How Johnson County compares: Easiest Cities to Build an ADU·Compare Johnson County to another location·View the Iowa accessory structures overview
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