ADU rules in Johnson County, IA — also called accessory dwelling unit regulations or granny flat ordinances — cover setbacks, owner-occupancy, parking, and permit requirements.
Correcting a common belief: since July 1, 2025 Iowa has a statewide ADU mandate. Senate File 592 requires Johnson County and every city to allow at least one accessory dwelling unit on a single-family lot, with no owner-occupancy condition.
The 2025 Legislature enacted Senate File 592, adding Iowa Code section 331.301(27) for counties and 364.3(20) for cities. Effective July 1, 2025, Johnson County and Iowa City, Coralville, and North Liberty must permit one ADU on the same lot as a single-family residence, attached or detached, without discretionary review or a hearing. The unit cannot exceed 1,000 square feet or 50 percent of the house, whichever is larger. Local governments cannot require the owner to live on-site, mandate extra parking, or impose appearance standards stricter than for a single-family home. A building permit still applies, and tornado-safe design is worth planning for.
Building or occupying an ADU without the required building permit is a code violation. The county or city can require the unit inspected, brought to building-code standards, or removed.
Other ordinances people look up for this city. Green dot = verified primary-source excerpt.
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See how Johnson County's adu rules rules stack up against other locations.
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