Is an Engineer Stamp Required for My ADU Project in Illinois?

Illinois' municipalities — Chicago, Aurora, Naperville, Springfield — review structural drawings for accessory dwellings, and the PE seal is the standard requirement. The seal must show the engineer's name, Illinois PE number, and date, and the engineer must be in responsible charge of the engineering. Detached ADUs, garage conversions, and coach houses are treated the same at the structural level. In Chicago, the Department of Buildings adds its own plan-review layers beyond the seal itself. Owner-builders don't escape the requirement: the department asks for the seal regardless of who drafted the plans.

The engineer reviews the floor plan and elevations, designs the foundation below the deep frost line — Chicago's clay gets extra attention — sizes beams, headers, and the lateral system, and checks snow, wind, and seismic design. Southern Illinois seismic demand is checked where triggered. Garage conversions get extra scrutiny because the existing slab was rarely designed as a dwelling foundation. Bring the site survey, the geotechnical report, and the architectural plans. Frost-depth compliance is the item most ADU plans miss.

Use this checklist before engaging an engineer for your ADU: • Engineer of record holds an active Illinois PE license, verifiable on the state roster • Dimensioned floor plan and elevations ready for structural review • Site survey showing setbacks, easements, and existing structures • Frost-depth compliance verified for footings, plumbing, and equipment • Snow loads matched to the site jurisdiction • Garage conversions: existing slab evaluated for dwelling loads • Engineer confirms they'll respond to plan-check corrections on sealed sheets • Seal shows name, Illinois PE number, and date on every sealed sheet