ADU Rules & Cost in Illinois
A 700 sq ft detached ADU in Illinois typically costs $169,500–$277,000 ($242–$396/sq ft). Here is what the rules allow.
Local zoning governs — no statewide ADU lawIllinois ADU rules at a glance
This state has no statewide by-right ADU law, so accessory dwelling units are governed entirely by your city or county zoning code. Many cities still allow ADUs — check your local planning department for size, setback and parking rules.
- Statewide law
- No — local rules
- Max detached size
- Set locally
- Guaranteed minimum
- Set locally
- Setback
- Set locally
- Owner-occupancy
- Varies locally
- Approval time
- Set locally
Parking: Set by local ordinance — commonly 1 space per ADU unless near transit.
Estimate your Illinois ADU
Pre-set to Illinois. Adjust the type and size for your project.
Your property
ADU typeEstimated cost in Illinois
$169,500 – $277,000
≈ $242–$396/sq ft turnkey · midpoint $223,500
- $152,000
- Construction
- $40,200
- Design & permits
- $31,300
- Site work
- iSize set locally. This state leaves ADU size limits to your city — typical caps run 600–1,200 sq ft. Check your local code.
- iParking. Set by local ordinance — commonly 1 space per ADU unless near transit.
- iOwner-occupancy varies. Owner-occupancy rules are set locally in this state.
- ✓Lot has room. A 700 sq ft ADU uses about 12% of your 6,000 sq ft lot — comfortably within typical coverage limits.
Rule basis: Local zoning governs — rules vary by city/county
If you rent it out
$1,175–$1,925
est. rent/mo
8.3%
gross yield
12.9 yrs
payback
~$17,298/yr after a 7% vacancy allowance, before financing & maintenance.
Financed over 20 yrs at 7.5%, the build is about $1,801/mo — vs ~$1,550/mo in estimated rent.
Estimates are for planning only and are based on regional construction-cost indices and published statewide ADU statutes. Local ordinances, lot conditions and contractor pricing vary — always confirm with your city planning department and a licensed contractor.
Cost by ADU type in Illinois
| ADU type | 600 sq ft | $/sq ft |
|---|---|---|
| Detached new build | $145,000–$237,500 | $242–$396 |
| Attached addition | $132,000–$218,000 | $220–$363 |
| Garage conversion | $79,000–$145,000 | $132–$242 |
| Junior ADU (JADU) | $66,000–$119,000 | $110–$198 |
| Prefab / modular | $119,000–$198,000 | $198–$330 |
Financing your ADU
A handful of loan and grant programmes are built specifically for ADUs. These are informational links we don't currently earn a commission on — not paid placements.
RenoFi
Compare RenoFi ADU loan options →Renovation-specific loans (home equity loan, HELOC or cash-out refinance) that lend against your home's value after the ADU is built, so you can often borrow more than a standard HELOC allows.
Fannie Mae HomeStyle Renovation
See HomeStyle Renovation →A conventional mortgage that rolls ADU construction costs into a single loan, with down payments as low as 3% for qualifying first-time buyers.
CalHFA ADU Grant Program
Check CalHFA grant status →Reimburses up to $40,000 of pre-development costs (design, permits, soil tests) for California homeowners building an ADU.
Current round of funding is fully allocated — the page covers what to check for reopening.
Want financing options matched to your project?
Tell us roughly what you need to borrow and we'll point you to lenders who work with ADU projects.
ADU rules by city in Illinois
Estimates are for planning only and are based on regional construction-cost indices and published statewide ADU statutes. Local ordinances, lot conditions and contractor pricing vary — always confirm with your city planning department and a licensed contractor.