ADU Guide · Verified 2026
Building an ADU in Illinois
Illinois has no statewide ADU mandate, but Chicago dramatically expanded its own program in 2026, going from about 12% of the city eligible to about 60%. Multiple bills that would create a statewide floor are actively moving through Springfield, none had passed as of this research.
Quick Facts
Statewide floors set by No statewide mandate; Chicago's own Additional Dwelling Units Ordinance, expanded April 1, 2026. Your city may be more generous but cannot go below them.
The Three Types of ADU
A real, recent, dramatic expansion in Chicago specifically
Chicago's ADU pilot ran in just five designated areas, covering roughly 12% of the city, from May 2021 until the City Council voted 46-0 in September 2025 to make it permanent and citywide, effective April 1, 2026, expanding eligibility to roughly 60% of Chicago, a genuinely large jump in a single ordinance change.
A distinctive labor requirement most cities don't have
Contractors building a coach house in Chicago must participate in a federally registered apprenticeship program as of the same April 1, 2026 effective date, a real labor-standards condition layered directly onto the zoning ordinance.
Real legislative activity, still pending
Four separate bills, SB 4071, HB 5626, SB 3726, and HB 3552, are actively moving through the Illinois legislature and would each create some form of statewide ADU floor, but none had passed as of this research, meaning Illinois remains a fully local state for now despite the active push.
What people get wrong
The most common mistake is assuming Illinois's active legislative push means a statewide law already exists. It doesn't yet, and Chicago's own dramatic 2026 expansion is a local ordinance change, not a statewide one.
From Idea to Move-In: The Timeline
Who Administers It
Counties With Their Own Rules
These Illinois counties have verified, county-specific ADU rules that add to the state baseline:
Building It Yourself? Here's What You'll Need
Planning an ADU? Tools homeowners use in the design and layout phase:
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Check Your Local Rules
Your city may allow more than the state floor
State law sets the minimum; many Illinois cities and counties are more generous. Browse the full directory of 102 counties and every city in Illinois for local rules, fees, and building department contact details.
Browse the full Illinois directory →Not sure if your project needs a building permit, a zoning approval, or both? See our guide to building permits vs. zoning approval.
Official Sources
ADU law changes frequently and local ordinances add detail. This guide is general information, not legal advice, always confirm with your local building department. Reviewed by the StateDataIndex Editorial Team · Updated September 2026.