As more communities look to expand housing types, they may consider a simple idea: accessory dwelling units.

An accessory dwelling unit (ADU) is a second housing unit on a single-family residential lot. Commonly referred to as granny flats, coach houses, and in-law suites, ADUs are smaller, independent units with a full kitchen and bathroom. These separate living spaces can be located within the home, such as a basement apartment; attached to the home, such as a garage apartment; or detached from the home, such as a cottage.

ON TO 2050, the comprehensive plan for northeastern Illinois, calls for communities to align zoning, approval processes, building codes, and inspections to generate more housing options. Communities are motivated to approve ADUs because they are a convenient and affordable housing option that can also maintain neighborhood character. They provide housing opportunities for older adults, empty nesters, and young adults who want to live close to family members. ADUs help to meet the growing demand for housing, increase access to jobs and services, promote compact development, and support multigenerational living.

ADUs are growing in popularity both locally and throughout the state. Nearly a dozen communities in northeastern Illinois currently permit ADUs through their zoning ordinances or are considering amending their zoning ordinances to permit ADUs (see table below, Comparing ADU zoning codes in northeastern Illinois).

ADUs have been developed in communities bordering Chicago, like Evanston, and in outer suburbs, like South Elgin. In December 2020, Chicago repealed its ban on ADUs by approving a pilot program in five broad areas of the city. The pilot program took effect on May 1, 2021.

ADU codes and ordinances

There is no one-size-fits-all approach to allowing ADUs. Municipalities regulate ADUs through building and zoning codes. Communities typically tailor how they permit ADUs to meet local goals and fit with the local character. Zoning codes may address the following ADU standards:

Occupancy

Neighbors are frequently concerned that an ADU will not be cared for, or that an absentee landlord will rent out both the home and the ADU. To ensure the property is maintained, some municipalities require that the owner live in either the primary residence or the ADU. In other communities, only family members can live in ADUs.

Design

Communities may specify the form and appearance of ADUs to fit in with the character of existing homes. Zoning standards may require that the exterior materials of the ADU are compatible with the primary home, including siding material, window design, and roof shape.

Communities with larger homes may allow attached ADUs with a separate entrance, while communities with larger lots may allow detached ADUs by adding a new freestanding unit or converting a detached garage. Some communities allow both attached and detached ADUs.

Size

Communities can limit the size and height of an ADU to ensure that they are clearly secondary to the principal housing unit on the site. When communities allow detached ADUs, they can limit the total square footage or its coverage of the lot area. For an attached unit, communities can limit the total square footage or percentage of interior space.

Communities can also set the maximum number of bedrooms within an ADU. In municipalities with many housing types, these square footage and bedroom limits can vary based on the size of the lot or the zoning district of the home.

Parking

The potential impact on parking is a frequent concern, especially in communities where parking is scarce. Research suggests that ADU tenants tend to own fewer cars than other households do and may walk, bike, or use transit if those options are convenient.

Communities may require one parking space per ADU or per bedroom in an ADU. Not including parking requirements encourages people to develop ADUs.

Administration

The way in which ADUs are administered can impact their feasibility within a community. Some are permitted by right and only require building code and zoning compliance. Some communities categorize ADUs as a special or conditional use requiring a public hearing. Although the public hearing process allows for more in-depth review of projects, it can be time consuming and generally has additional requirements that can be a significant financial burden for homeowners. Allowing ADUs as a permitted use encourages people to develop them.

Although ADUs diversify the local housing stock, communities may be resistant to approving this type of housing. Density, overcrowding, and parking and traffic impacts are among the top concerns for local municipalities. While these concerns are valid, impacts from ADUs are generally minimal.

Within communities that permit ADUs, there must be property owners willing to make a financial investment and potentially serve as a landlord. They also must have property that can accommodate an additional dwelling unit that meets zoning bulk and setback requirements.

Because of the various conditions required to construct an ADU, they are generally built at a modest rate. As of May 2021, Oak Park has issued 10 ADU building permits since it adopted its ADU ordinance in 2018. Evanston has issued 10 permits since 2019.

It is critical to have a public conversation to ensure that ADUs are right for your community. Every municipality is different,and there is no one-size-fits-all policy.

Introduce the concept of ADUs during conversations around housing diversity and affordability. The next step is to update the municipality’s zoning code to permit ADUs. The Chicago Metropolitan Agency for Planning (CMAP) has assisted municipalities in drafting zoning codes that permit ADUs through its Local Technical Assistance program, most recently Park Forest and South Elgin. CMAP has also developed sample ordinance language that municipalities can use to permit ADUs.

For more resources and information about ADUs, visit:

• The ABCs of ADUs: A Guide to Accessory Dwelling Units and How They Expand Housing Options for People of All Ages, AARP

• AccessoryDwellingUnits.org, including national examples of completed ADUs, testimonials from homeowners, and model ordinance language for municipalities

• Granny Flat Academy