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Spring 2019 – Workshop #2: Tracking the Elusive ADU

On Wednesday 2/27, City Open convened once again at The Great Cities Institute at UIC. This week, we turned to a new technique – how to use data to count Accessory Dwelling Units (ADUs). Specifically, we sought to answer the following two questions: How many ADUs are there? And how can we estimate the number of new ADUs that could be built if they are re-legalized?

Season Theme: Accessory Dwelling Units

Workshop Method: Using Data to Count ADUs

In the first part of the workshop, Steven Vance presented his counting technique for finding Chicago ADUs with the reminder that “what’s counted matters.” The urgency to understand ADU numbers comes from the 5 year housing plan adopted in November 2018 by Mayor Rahm Emanuel and the City Council which lists #1.9: “Create affordable housing through Accessory Dwelling Units (ADUs).” Before any new measures are put into place, it’s important to understand the current state of ADUs (or coach houses) in Chicago and what could come from policy changes.

Photo by Paola Aguirre

The first part of Steven’s presentation asked: How common are coach houses? He explained that there are three types of data that could be used to count couch houses:

  1. Assessor Data: Every 3 years the Cook County Assessor’s office “re-assess” the value of a property. However, line #2-11 which reads: “Apartment building with 2 to 6 units” is flawed because it does not include coach houses.

  2. Building Footprints: Presented via online mapping sites like Google Maps and OpenStreetMap, this method is generated in a few ways. The first is computer assisted mapping but this method can be unreliable. The second (and what could arguably be the coolest) method involved the city of Chicago hiring pilots to fly planes and take pictures of the built environment. These photos were then manually traced into a GIS program, ensuring that no buildings were missed. This stroke of ingenuity means that, in addition to being less reliable in general, computer assisted mapping is also less useful in Chicago. Proving that people (and planes) can be more reliable than computers (for now).

  3. Duplicate Address Method: Using a GIS program, Steven found 3,138 coach houses in Chicago. He then verified these with satellite images and also by checking for the number of mailboxes assigned to a lot. Adding another layer of sleuthing to the mix, Steven noted that Google Streets sometimes drives through alleys and you can look for windows above garages. He’s had a 77% success rate in confirming the duplicate address method. If you feel inspired to join Steven in this detective work, make sure to be on the lookout for coach houses. Anytime you find one, make sure to note the address of the front house or any address you can find. Then search OpenStreetMap or Chicago Cityscape to see if it’s already on the map. If it’s not in there, send Steven an email with the address so he can check on it and rerun his analysis.

Photo by Lindsey Conklin

The second part of Steven’s presentation asked: How many coach houses could be built? To come up with an estimate for this question, he put forth two potential policy parameters:

  1. Policy A: What if… “any single family lot could have one ADU”? For this new policy measure, we would need to consult the city zoning map and the Cook County parcel map to estimate new ADUs. Using these maps, he isolated 7,064 residential parcels (excluding condos) of which 47% have a 2-6 flat. The remaining are single family homes (not condos or row houses) and would be eligible for ADUs. This new policy, then, would result in 3,634 new ADUs being allowed.

  2. Policy B: What if… “any residential-zoned lot could have two ADUs”? Using the same two maps as the previous measure and similar math, this new policy would result in 14,128 new ADUs being allowed. These numbers, of course, are contingent upon a property owner choosing to build an ADU.

Once draft legislation is put forward this March, we can start running tests on the number of ADUs that would be allowed within the new policy. Steven also presented example policy from other places. Notably, in Oak Park, the policy requires a minimum 6000ft lot size and an existing garage. This limits potential ADUs to only 10-12% of property owners in Oak Park.

After Steven’s presentation, the group split into two to consider: What kind of resources can we create as a group for website or for a community clinic?

Photo by Paola Aguirre

We split in half and focused on workshopping two potential interventions:

Group 1: Resource Guide that Troubleshoots Questions from Residents

-If we’re trying to make a case for ADUs, we need to be able to communicate this to everyone. How can we communicate this to an 8 year old all the way up to an 80 year old?

Group 2: Policy Lobby Document with Briefing and Policy Recommendations

-Could we get this done in time to present to run-off Alderman and Mayoral candidates?

Come join us for the next City Open Workshop as we continue to flesh out these ideas and resources for communities and residents of Chicago.

Special thanks to Steven Vance for presenting his data methods for ADUs. Make sure to check out his petition to re-legalize coach and rear houses and newly legalize ADUs in Chicago.

You can find notes from this workshop here.


LINDSEY CONKLIN is an anthropologist and researcher who works at the intersection of social impact design, the built environment, and community development.

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