Iowa ADU law: building an accessory dwelling unit

What state law lets you build

Senate File 592 passed the Senate 49-0 and the House 89-7. The governor signed it on May 1, 2025 (2025 Acts, chapter 38), and it took effect July 1, 2025.

It added the same ADU rule to the law for cities, Iowa Code 364.3(23), and for counties, Iowa Code 331.301(29). Each says the city or county "shall allow a minimum of one accessory dwelling unit on the same lot as a single family residence."

The law defines an ADU as "an additional residential dwelling unit located on the same lot as a single family residence that is either attached to or detached from the single family residence." A detached ADU "includes being part of any accessory structure such as a detached garage."

The ADU must meet all applicable building regulations. [1][2][3][5]

Who is covered: cities and counties

Iowa's law covers both cities and counties. If your home is inside city limits, your city's rules must follow Iowa Code 364.3(23). County zoning powers apply only to land "outside of the corporate limits of any city," and county rules must follow Iowa Code 331.301(29). The ADU rules in the two sections are nearly identical.

Zoning under Iowa's county zoning chapter applies to a county "at the option of the board of supervisors." If you live outside city limits, ask your county planning and zoning office which rules apply to your lot.

Since July 1, 2026, SF 2369 says the ADU rule applies "within a zoning district where a single family residence is an allowed principal use." [1][2][4][6][7][8]

How big your ADU can be

The law says an ADU "shall not exceed one thousand square feet or fifty percent of the size of the single family residence, whichever is larger." For example, with a 2,400 sq ft house, 50% is 1,200 sq ft, so the cap is 1,200 sq ft. With a 1,500 sq ft house, 50% is 750 sq ft, so the cap is 1,000 sq ft.

SF 2369, in effect July 1, 2026, adds two measuring rules:

• "Size" of the house means "the gross living area of the primary dwelling unit excluding garages, decks, and unheated porches."
• "The size calculation of an accessory dwelling unit shall exclude unfinished basements."

A city or county may adopt rules that are more generous. [1][2][4][8]

What cities and counties can't require

Unless the ADU section itself or another state law allows it, a city or county may not impose:

• placement or appearance rules for the ADU that are stricter than for the house, including height limits, setbacks, minimum lot sizes, frontage, lot coverage, density, and aesthetic or architectural standards;
• a rule that the ADU match the house's exterior design, roof pitch or finishing materials;
• more parking than is required for the house alone, or a fee in place of parking;
• a rule that the owner live on the property, a family, marriage or employment link between the people in the two units, or limits on who can live in the ADU based on income or age;
• new or separate utility lines, unless full utility access with separate metering for billing can't be provided to the ADU;
• a different impact fee structure or development standard for the ADU than for the house on the same lot;
• street or sidewalk improvements beyond those required for the house.

Rental rules for an ADU can't be stricter than those allowed under other listed state laws on rentals. [1][2]

How fast the city or county must decide

A city or county "shall approve an accessory dwelling unit permit application that meets the requirements" of the law "without discretionary review or hearing and consistent with the time frame assigned to the approval of a single family residence."

Your ADU application can't have a review timeline longer than the normal review schedule for a single-family home. If the city or county denies the permit, it must give you the reason in writing and "include any remedy necessary to secure approval."

For a general walk-through of plans, permits and inspections, see our ADU permit process guide. [1][2]

HOAs, deed restrictions and historic districts

Iowa's law lets some private rules still block an ADU. An ADU "shall be prohibited or limited only to the extent that" a state historic building code restriction adopted by the city or county, "a deed restriction, or a rule of a common interest community" limits or prohibits it. If your home is in an HOA or has deed restrictions, read them before you plan an ADU.

A city or county also can't apply ADU rules that are stricter in a common interest community than for a single-family home.

Historic districts. SF 2369, in effect July 1, 2026, adds a historic-area rule. In a county, an ADU proposed for a lot in a historic preservation district "shall be regulated only to the extent that" the historic preservation commission finds it "would be incongruous with the historical, architectural, archaeological, or cultural aspects of the district," and records its reasons. In a city, an ADU proposed for a lot in an area designated as an area of historical significance "shall be regulated to the extent that the city or commission" makes that finding and records its reasons. [1][2][4]

Manufactured homes and mobile homes as ADUs

If you use a manufactured home or a mobile home as an ADU, it must be converted to real property by being placed on a permanent foundation and assessed for real estate taxes. [1][2]

Renting out your ADU

The ADU law says rental rules for an ADU can't be stricter than those allowed under other state laws. For short-term rentals of 30 days or less, Iowa Code 414.1 says a city "shall not adopt or enforce any regulation, restriction, or other ordinance, including a conditional use permit requirement, relating to short-term rental properties within the city," except for listed purposes such as fire and building safety, sanitation, noise, property maintenance, nuisances and an emergency contact. A city may not require a license or permit fee for a short-term rental. Iowa Code 331.301(18) has a similar rule for counties.

Ask your city or county which rental rules apply before you list the ADU. [1][2][9]

If your city or county says no

Both code sections say a local ordinance, motion, resolution or amendment "regulating accessory dwelling units in a manner that conflicts with this subsection is void." A city or county may still adopt rules that are more permissive.

If your application is denied, the city or county must tell you why in writing and what you'd need to change to get approval. If its reason seems to conflict with the state law, ask the planning or zoning office in writing which rule it is applying and how that rule fits with Iowa Code 364.3(23) (cities) or 331.301(29) (counties). Building regulations, deed restrictions and HOA rules can still limit an ADU, so the answer may depend on your lot.

The law does not name a state agency that takes complaints. [1][2]

Iowa laws to know

• SF 592 (2025), Iowa Code 364.3(23)

  Requires every city to allow at least one ADU on a lot with a single-family home, sets the ADU's maximum size, and limits parking, owner-occupancy rules, fees and review times. [1][3][5]

• SF 592 (2025), Iowa Code 331.301(29)

  Puts nearly identical ADU rules on every county. [2][3][5]

• SF 2369 (2026)

  Limits the rule to zones where a single-family home is an allowed principal use, defines how size is measured, and adds a historic-district exception. [4][8]

Questions people ask