ADU Design in Altamont, IL

ADU Design in Illinois: what governs your project

Illinois has no statewide ADU statute; ADU rules are set by municipalities. Chicago has run an ADU pilot program and adopted ordinances allowing additional dwelling units in select areas, but most suburbs set their own, often restrictive, rules.

Energy code

Permitting is local, with zoning review, setbacks, and building code compliance. Cook County and collar-county jurisdictions vary widely on size caps, owner-occupancy, and utility requirements.

Permitting & plan check

Chicago's Department of Buildings runs its own permitting process, including a self-certification program that allows licensed architects and structural engineers to certify code compliance on qualifying projects. Outside Chicago, permits and inspections are handled by municipal and county building departments. Illinois has no single statewide commercial plan review; authority sits with local jurisdictions.

Good to know

• Illinois is one of the few states with a separate Structural Engineer (SE) license, required for the structural design of certain higher-risk structures.
• Southern Illinois sits near the New Madrid Seismic Zone, so seismic design categories there are notably higher than in the Chicago area.
• Chicago's downtown high-rises famously bear on caissons drilled to bedrock through deep layers of soft clay.

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