Permit or Not · All projects · Add a shed or small outbuilding · Illinois

Do I need a permit to add a shed or small outbuilding in Illinois?

It depends on the specifics

One-story detached sheds are typically exempt up to 200 square feet under the residential code — but the number people quote is 120, and both are right depending on which code your jurisdiction adopted.

How Illinois decides

Code adoption
Home rule — local adoption
Edition on file
No statewide edition
Local amendments
Permitted
Verified county offices
23 of Illinois’s counties

Illinois does not bind its local governments to one statewide building code. Cities and counties adopt their own, which means two neighbouring towns can be on different editions with different thresholds, and neither is doing anything unusual. The rule below is the model-code baseline and the starting point for a conversation with your building department, not a substitute for it.

There is no single statewide code edition in Illinois to point at. Which edition applies to you is decided by whichever city or county issues your permit, and it is a fair first question to ask them.

On file for Illinois: No statewide adoption; local jurisdictions vary.

Adoption data from Code Adoption by State (Code Check), retrieved 2026-08-09, checked for Illinois on 2026-08-09. That source gives the edition and whether adoption binds statewide. It does not give per-project thresholds, and neither do we — the thresholds on this page are the model code's. Code Adoption by State →

What changes the answer

These thresholds come from the model code. Illinois lets its cities and counties amend the code they adopt, and an amended threshold is usually where people get caught.

IRC R105.2 exempts one-story detached accessory structures not exceeding 200 sq ft in the 2015, 2018 and 2021 editions. Earlier IRC editions and the IBC at 105.2 both say 120 sq ft, which is why the smaller figure is still quoted everywhere and is a common local amendment.

Why it matters

This is the single most contradictory number in residential permitting, and the contradiction is real rather than a matter of someone being wrong. Assuming the more generous figure is exactly how people end up with a structure that has to come down.

Confirm the requirement with the Illinois city or county building department that would issue the permit before you start.

Illinois county building departments we have verified

Permits in Illinois are issued by city and county building departments, not by the state. These 23 county permit pages have each been opened and checked, including a check that the page belongs to a county in Illinois — one in ten links in our first pass was a building department in another state entirely, so every entry here names the wording that identified it.

County offices. If your address sits inside a city or town, the city issues the permit and the county page will send you back out again. Checked 2026-08-25. We can resolve a ZIP to a proven county page for 587 Illinois ZIP codes; outside those, the checker falls back to a search rather than a guess.

This is the typical rule, not your answer

Nothing here is a permit decision, and it is not legal or engineering advice. No page can settle this for your address — permitting in Illinois is run by individual city and county building departments, and the one that covers your property is the only one whose answer binds anyone. Put a ZIP in and we will name the office that decides, and show you the county permit page where we have proven one.

More on add a shed or small outbuilding

The full add a shed or small outbuilding guide covers why the thresholds sit where they do, what catches people, and the narrower versions of this question — extending rather than building, repairing rather than replacing, and the cases that sit right on a limit.

Add a shed or small outbuilding in other states