ADU laws in Iowa

In plain words

Iowa has a statewide ADU law. An ADU, or accessory dwelling unit, is a smaller second home on the same lot as a house. It can be attached to the house or stand on its own, including space in a detached garage. Since July 1, 2025, every Iowa city and county must allow at least one ADU on a lot with a single-family home. Since July 1, 2026, this applies in zoning districts where a single-family home is allowed. Zoning is the local rulebook for what can be built where. The state floor for size is 1,000 square feet, or half the main house if that is bigger. Towns may allow more. Towns cannot require the owner to live on the property. That rule is called owner occupancy. Towns also cannot demand extra parking. A setback is the required distance from a property line. Towns cannot make the ADU's setbacks, height or lot-size rules stricter than the house's. They also cannot charge it different impact fees, which are one-time charges for new building. A permit that meets the rules must be approved without a public hearing or a board vote. This is called by-right approval. If a town says no, it must explain why in writing and say how to fix it. Some limits still apply. Building codes still apply. A homeowners association (HOA) rule or a deed restriction, which is a limit written into your property's deed, can still block an ADU. In historic districts, a commission may limit one if it records its reasons. You may be able to build only if local rules and your property allow it.

The rules at a glance

What to do next

1. Look up your zoning district and confirm a single-family home is an allowed use there; the state guarantee only covers those districts.
2. Read your deed and any HOA covenants, because private restrictions can still ban an ADU in Iowa.
3. Ask your city or county planning office for its current ADU ordinance and permit checklist; many rewrote their rules in 2025 and again after July 1, 2026. Ask whether historic district review applies to your lot.
4. Talk to your utility providers early about service and metering; a town can require separate lines only if the ADU cannot get full service with separate metering through shared lines.
5. Design within the size limit (1,000 sq ft or 50% of the house's living area, whichever is larger) and the same setback and height rules as the house, and plan for building code inspections.
6. If you plan to rent the ADU, check local rental registration and inspection rules. If a permit is denied, ask for the written reason and the fix the law requires the town to give.

What your town still decides

Cities and counties still write their own ADU ordinances, but any rule that conflicts with the state law is void, and towns may always be more generous. Building codes (Iowa Code ch. 103A) still apply. Deed restrictions and homeowners association (common interest community) rules can still ban or limit ADUs, but a town may not apply stricter ADU rules inside an HOA community than elsewhere. A manufactured or mobile home used as an ADU must sit on a permanent foundation and be taxed as real estate. Many cities rewrote their codes after the 2025 law and again for the July 1, 2026 changes (Des Moines amended its zoning chapters in late 2025; Ames drafted changes in May 2026). Local rental licensing and inspection codes still apply to rented ADUs. Some cities add limits the statute does not mention, such as Ames limiting an ADU to one bedroom and not allowing ADUs carved out of an existing house; whether such limits survive a challenge under the state law has not been tested. Rural lots may still face septic and well limits.

Examples of local rules

• Des Moines: One ADU per lot, accessory to a one-household home, in the N, NX, A, DXR, RX1 and RX2 zoning districts. It can be inside the house or in a detached building in the rear yard, at least 5 feet from lot lines, no taller than the height limit for a house on that lot, and up to 1,000 sq ft or 50% of the house's floor area, whichever is larger. No extra parking is required. A rental certificate is needed if someone outside the owner's close family lives there, and the city offers a 10-year tax abatement on the value an ADU adds. Source
• Ames: A May 2026 staff proposal to match the July 1, 2026 state changes allows one newly built attached or detached ADU (or a converted accessory building) on single-family lots, using the house's height, setback, lot and design rules. It keeps a one-bedroom limit, requires public water and sewer and a permanent foundation, adds no parking requirement beyond the house's, and requires rental registration if rented. Final council action should be checked. Source
• Iowa City: A 2023 code update allows ADUs with any detached or attached single-family or duplex home in any zone that allows residential living, up to the lesser of 1,000 sq ft or 50% of the main building, with no extra parking. That update kept an owner-on-site rule, and its 'lesser of' size cap is tighter than the state's 'whichever is larger' floor; the 2025 state law voids both to the extent they conflict. Check the current code for later changes. Source
• Linn County (unincorporated areas around Cedar Rapids): One ADU per parcel with an existing single-family home, in a detached unit, an accessory building, a tiny house or manufactured home on a permanent foundation, or part of the house. The county allows up to 1,200 sq ft or 50% of the house, whichever is larger, which is more generous than the state minimum. A minor site plan is required. Source

What we’re still confirming

• State law protects ADUs attached to or detached from the house, but whether it protects a basement or other inside conversion isn't settled; Ames reads it as not covered.
• Whether cities may add limits the law doesn't list, like Ames's one-bedroom cap, hasn't been decided by a court.

Laws, bills and citations

This is general information, not legal advice. Your town, village, city or county decides what can be built on your property. If something here looks out of date, email the team.