ADU Cost in Illinois in 2026

ADU costs in Illinois range from $62,000 for a basic basement conversion in Springfield to $358,000+ for a high-end coach house on Chicago's North Side. The most common Chicago build — a 600 sq ft mid-range coach house — costs $150,000 to $250,000. Basement conversion ADUs cost $65,000–$120,000 and offer the best ROI entry point. Union labor rates in Chicago run 15–25% above the national average, driving the premium over other Midwest markets.

Chicago's permanent ADU ordinance took effect April 1, 2026, expanding the pilot program that had been running since 2021 and 2022. Under the permanent ordinance, all RT-zoned and RM-zoned lots citywide are eligible for ADU construction without requiring aldermanic opt-in. RS-zoned (single-family) lots still require aldermanic approval in most cases. Both basement/attic conversion ADUs and detached coach houses are permitted under the ordinance.

A coach house is Chicago's term for a detached ADU — a freestanding secondary dwelling unit typically accessed from the alley at the rear of a lot. Coach houses are historically common in Chicago's greystone and two-flat neighborhoods and are the most distinct ADU type in the city. New coach house construction costs $150,000–$350,000 and can command $1,500–$2,400/month in rental income. Chicago limits coach houses to 1,000 sq ft maximum.

Chicago's ADU ordinance explicitly prohibits short-term rental (any lease under 30 days) in all ADU units. This was a deliberate policy decision to prevent ADUs from being used as de facto hotels rather than as long-term housing for Chicago residents. Violating the STR prohibition can result in fines and revocation of your ADU permit. If short-term rental income is a goal, a Chicago ADU is not the right investment vehicle.

Chicago sits on clay soil with a high water table — a combination that makes basement water intrusion a near-certainty without proper protection. Chicago also has a combined sewer system that can back up during heavy rain events. Chicago basement ADU builders universally install interior drain tile, sump pumps with battery backup, vapor barriers, and backwater valves ($1,500–$3,000) as standard practice. Budget $8,000–$20,000 for waterproofing in addition to the base conversion cost.

A mid-range Chicago basement conversion costs $80,000 to $120,000 and generates $1,200 to $2,000 per month in garden unit rental income. Cash-on-cash ROI runs 10–14%. A coach house costs $160,000 to $250,000 and generates $1,500 to $2,400/mo, producing 8–10% ROI with a payback period of 10–14 years. Chicago ADUs add an estimated 64% value premium to properties with legal secondary units per Chicago market data.