Accessory Housing

When Des Moines revamped its PlanDSM zoning code in 2019, it opened up a new opportunity for accessory household units (AHU).

You might be familiar with AHUs as Accessory Dwelling Units (ADU), guest houses, granny flats, in-law units, alley flats, coach houses, or casitas. AHUs can be built into an existing house or garage, built as an attachment building to your house, or built as a separate building behind your house.

Accessory Dwelling Unit (ADU) Types

Ready to pursue an AHU?

Schedule a pre-application meeting with Development Services staff to discuss what you'll need to get started.

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Why Build an AHU?

From the City's perspective, AHUs add much needed opportunities to increase available housing options throughout Des Moines neighborhoods. They also add opportunities for homeowners to increase their property value, share the costs of homeownership with a renter, or provide accommodations to friends, family members, and in-home helpers.

An accessory household unit might be a good fit for you if:

  • You're looking to downsize, but you don't want to leave your neighborhood.
  • You have a family member who needs in-home care, but would prefer their own space. A possible alternative to an assisted-living facility or nursing home.
  • You have a nanny, au pair, or grandparent assisting with childcare and want to provide them their own housing accommodations.
  • You work from home, but would prefer to expand to a separate office or studio space.
  • You want to provide an opportunity for rental housing as a way of cost sharing or to generate supplemental income.

 

Where Can I Build an AHU?

Currently, PlanDSM - Chapter 134 provides you the right to build an accessory household unit if your property is located within the following areas:

  • A, DXR, RX1, and RX2 zoning districts
  • N and NX zoning districts that allow two, three or four household units (i.e. N1-2, N2-2, N3-2, N4-2, N5-2, N1-4, N2-4, N3-4, N4-4 and N5-4, NX1, NX2, NX2a and NX3 districts)
  • Any N and NX districts, regardless of number of household units on the lot, if the lot is within ¼ mile of an existing DART transit route

If your property is located within an N district that only allows a single household unit and there's no DART route within a quarter mile, you have the option to request a conditional use approval by the Zoning Board of Adjustment to build an AHU.

Some zoning districts come with additional development standards that need to be met.

Not quite sure what all that means? Don't worry. You can check how your property is zoned by entering your address into the Show Me My House tool, and you can contact Development Services staff at (515) 283-4192 or planning@dmgov.org to help clarify what's allowed in your district.

What Types of Accessory Housing Are Allowed in Des Moines?

Accessory household units can be created in many different shapes and sizes, but should be designed with the existing neighborhood in mind and be able to discreetly blend in.

  • A detached AHU is a stand-alone home on the same lot as a larger, primary dwelling. Examples include backyard bungalows and converted outbuildings.
  • An attached AHU connects to an existing house, typically through the construction of an addition along the home’s side or rear. Such units can have a separate or shared entrance.
  • A garage AHU makes use of an attached or detached garage by converting the space into a residence. Other options involve adding a second-story AHU above or beside a garage or building a new structure for both people and cars.
  • An internal AHU is created when a portion of an existing home is partitioned off and renovated to become a separate residence. This can be a lower-level AHU created through the conversion of a home’s existing basement (provided that height and safety conditions can be met).

 

How Much Does an AHU Cost?

The costs of AHUs vary depending on size, type, and level of finish. AHUs tend to cost more on a per-square-foot basis than a new home due to inefficiencies of scale.

The least expensive AHUs tend to be garage conversions or interior conversions. These tend to be in the $50,000 - $100,000 range, often involving some amount of “sweat equity” by the owner. The more expensive ones tend to be new detached structures or basement replacement house lifts.

Although the bulk of these costs are for construction costs, they also include some other costs such as:

  • Design and architect fees
  • Building permits and/or impact fees
  • Lender fees
  • Other professionals-based specific needs of your project (i.e. surveyor, structural engineer, environmental assessment/clean-up, project manager)

 

AHUs often are created by individual homeowners and financed through some combination of savings, second mortgages, home equity lines of credit (HELOC), and/or funds from family members. AHUs may also be financed using any loan designed for home addition or renovation projects, such as Fannie Mae’s “HomeStyle” renovation loan or FHA’s 203k rehab loan.

If your AHU plans require additional utility hookups for water, sewage, or electricity, that will add additional costs that need to be factored in. Before submitting your application, you'll need to check with your utility companies (Des Moines Water Works, Mid-American, cable, phone, etc.) to determine what will be required to provide service to your AHU.

You also should consider the impact of increasing your property value. To help ease the tax burden and encourage more of these housing opportunities, the City of Des Moines currently offers a 10-year tax abatement on the value an AHU adds to your property. Contact Development Services staff at (515) 283-4192 or planning@dmgov.org for more information.

What Does Des Moines Municipal Code Require?

  • Number Allowed: No more than one AHU is permitted on a single lot.
  • Occupancy: You must live on the property if you'll be renting out. Whether it's the primary house or the accessory household unit is your choice.
  • Location: AHUs may be located within the primary house or in a detached accessory building. Detached AHU buildings must be located in the rear yard.
  • Setback: AHU buildings shall be setback a minimum of five feet from any lot line.
  • Height: The maximum height is 17 feet, except where a higher maximum height is otherwise expressly permitted.
  • Roof Type: Roof type should match that of the primary house.
  • Materials: Materials shall match those of the primary house.
  • Character: AHU buildings shall be compatible with the color and character of the primary house on the lot.
  • Maximum Size: The lot coverage of all accessory structures on a lot, including an AHU building, may not exceed 576 square feet or 25% of the rear yard, whichever is greater. The floor area of an AHU must not exceed 50% of the floor area within of the primary house.
  • Parking: One additional off-street parking space must be provided per AHU (regardless of the type of AHU).
  • Entrances: Only one entrance to a house containing an AHU may face the street.

 

As with most major development and renovation projects in Des Moines, building an AHU requires permits from the Permit & Development Center before work can begin. During your pre-application meeting, Development Services staff will help you work through your AHU plans to make sure they meet municipal code requirements and have the permits they need.

You can schedule your pre-application meeting by calling (515) 283-4192 or emailing planning@dmgov.org.


Questions and Answers

A:  The floor area of an AHU must not exceed 50% of the floor area of the primary house. If the AHU is detached, it may not exceed 576 square feet or 25% of the rear yard, whichever is greater. The rental code includes square footage minimum based on the number of occupants that would limit how many individuals can reside in an AHU if rented. Applicants will be advised when pulling building permit of rental code square footage requirements.

Examples

The following information provides examples of the size an AHU would need to be to comply with the rental code (International Property Maintenance Code) based on different occupancy loads and unit configurations.

  • A single occupant efficiency requires 160 square feet in area that includes a 40 square foot bathroom. Additional floor area would be necessary for an HVAC system and a water heater. Consideration would also be needed for a cooking area with a minimum clear working area of 30 inches in front.
  • A double occupancy efficiency requires 220 square feet in area that includes a 40 square feet bathroom. Additional floor area would be necessary for an HVAC system and a water heater. Consideration would also be needed for a cooking area with a minimum clear working area of 30 inches in front.
  • A triple occupancy efficiency requires 360 square feet in area that includes a 40 square feet bathroom. Additional floor area would be necessary for an HVAC system and a water heater. Consideration would also be needed for a cooking area with a minimum clear working area of 30 inches in front.
  • A one-bedroom, single occupancy, unit with a kitchen and a bathroom would need 300 square feet of floor area. This calculation uses a floor area for the kitchen at 70 square feet and the bathroom at 40. Additional space would need to be provided for HVAC and water heater.
  • A one-bedroom unit, double occupancy, with a kitchen and a bathroom would need 330 square feet of floor area. This calculation uses a floor area for the kitchen at 70 square feet and a 40 square feet bathroom. Additional space would need to be provided for HVAC and water heater.
  • A two-bedroom unit, for up to 4 occupants, with a kitchen and a bathroom would need 440 square feet of floor area. This calculation uses a floor area for the kitchen at 100 square feet and the bathroom at 40. Additional space would need to be provided for HVAC and water heater.
  • A three-bedroom unit, for up to 6 occupants with a kitchen and a bathroom would need 690 square feet of floor area. This calculation uses a floor area of 100 square feet for the kitchen and 40 square feet for the bathroom. Additional floor would be necessary for an HVAC system and a water heater.

Considerations

Industry standards for kitchen and bathroom sizes were used for these examples. The area might be able to be reduced based on the equipment used. There must be air flow around the furnace and water heating units, and it will depend on the type of fuel source to determine appropriate area needed for installation. Baseboard electric heat or a mini split would reduce the area needed for a heat source. On demand hot water system could change the floor plan.

  • Plans will need to account for light and ventilation requirements of the building code.
  • Habitable space must provide 7 feet in any plan dimension. Habitable rooms are defined as a space in a structure for living sleeping, eating, or cooking.
  • Ceiling height must be 7 feet. With an exception for sleeping rooms with a percentage of a sloped roof, a minimum clear ceiling height of 7 feet over not less than- one third of the area.
  • Basements ceiling heights for laundry, study or recreation is set at 6 feet 8 inches. That might make it difficult to put a bedroom in some basements.
  • Kitchens must have a minimum 3 feet of clear passage.

A:  Detached AHUs must be located in a rear yard and setback at least 5 feet from all property lines. Detached AHU may not exceed 576 square feet or 25% of the rear yard. This limits the amount of surface area that is covered (impervious) and not available for stormwater absorption. The impact will be no greater than the impact of a property having one or more accessory buildings (aka, garages and sheds), which is currently allowed.

A:  City code requires 1 off-street parking space per household unit. This includes AHUs, houses, duplexes, and other residential type uses. A lot with a house on it is currently required to have a minimum of 1 off-street parking spaces. A lot with a house and an AHU would be required to have a minimum of 2 off-street parking spaces. This is the same requirement that is applied to a lot with a traditional duplex, which requires a minimum of 2 off-street parking spaces.

A:  The property that contains the primary house and AHU will have a single address with each building being noted as Unit 1 and Unit 2.

A:  A rental certificate is required if the AHU is occupied by someone that is not a member of the property owner’s nuclear family (i.e., parents, grandparents, children, grandchildren). This is the same standard that applies to all housing. One of the units must be occupied by the owner of the property regardless of familiar status. The AHU and primary house can be used by the same occupant should the owner of the property no longer reside in one of the units but cannot be rented and occupied separately.

A:  It depends on the location, depth and capacity of existing utilities in relation to the location of the proposed AHU. Homeowners should consult architects, general contractors, licensed plumbers and licensed electricians for to develop cost estimates of each alternative.

A:  Pets are allowed in AHUs if they are licensed in accordance with city code. They are treated no differently than pets are for separate units in a duplex.

A:  The Des Moines Public Works Department would provide trash and recycling bins for both the house and the AHU.

A:  Accessory Housing Units are taxed as residential property. The County Assessor will ultimately assign the assessed value of the Accessory Housing Unit similar to assessment rolls for a primary dwelling.

A:  Please see the following links: