What your community needs to know about accessory dwelling units

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.

What are the benefits of ADUs?

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.

Where are ADUs currently allowed?

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 In December 2020, Chicago repealed its ban on ADUs by approving a pilot program in five broad areas of the city.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.

Map of communities that allow ADUs: Antioch, Bull Valley, Chicago, Evanston, Homewood, Lake Bluff, Northbrook, Oak Park, Park Forest, South Elgin, Wilmette, and Woodstock. ADUs are inder consideration in Naperviille and River Forest

ADU codes and ordinances

Updated May 2023

AntiochOrdinance 19-12-43
Bull ValleyZoning code, section 11A – Accessory Apartments
ChicagoAdditional Dwelling Unit Ordinance
EvanstonADUs in Evanston
HomewoodZoning ordinance, section 44-04-14 Accessory Use Standards
Lake BluffMunicipal code section 10-5-15: Accessory Dwelling Units
NorthbrookOrdinance 2022-12
Oak ParkZoning ordinance, section 9.3 Accessory Structures and Uses
Park Forest§III-4.D. Accessory Dwelling Units and Home Occupations
South ElginUnified development ordinance §156.07.D.4. Accessory dwelling units
WilmetteZoning ordinance section 30-12.3(a) Accessory Living Unit
WoodstockUnified development ordinance, 7B.3.2 Accessory Uses and Structures

Key considerations for ADUs

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:


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.


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.


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.


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.


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.

Comparing ADU zoning codes

June 2021

Bull ValleyEvanstonPark ForestElgin
TypeAttached and detachedAttached and detachedAttachedAttached and detached
Maximum number
per lot
1 ADU per single-family home1 ADU per single-family home1 ADU per single-family homeSame as the number of principal dwellings permitted
LocationSubject to side and rear setback requirementsSubject to side and rear setback requirementsSubject to side and rear setback requirementsSubject to rear setback requirement if ADU is detached
Maximum sizeNo more than 30% of the net floor area of the principal dwelling; 2 bedrooms1,000 square feet (detached);
1 full floor (attached)
Not more than 40% of the gross floor area of the principal structure; or 800 square feet900 square feet
OccupancyProperty owner must live in principal dwelling or ADUNone requiredProperty owner must live in principal dwelling or ADUNone required
Parking1 space per ADUNone required1 space per ADU1 space per ADU
Design requirementYesYesNone requiredYes
Admin.Allowed by right with building permit approvalAllowed by right with building permit approvalAllowed by right with building permit approvalAllowed with special-use permit approval

What are the challenges with ADUs?

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.

How can my community take the next step?

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 the Local Technical Assistance program. CMAP has also developed sample ordinance language that municipalities can use to permit ADUs.

For more resources and information about ADUs, visit: