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2026 Cost Guide · Updated June 2026

ADU Cost in Illinois — Chicago, Springfield, Peoria & Statewide

Chicago just ended a nearly 70-year ban on coach houses — its new citywide ADU ordinance takes effect April 1, 2026. City-by-city cost data across Chicago, Springfield, and Peoria — built from permit records, contractor data, and regional labor rates.

$80K–$300KChicago ADU project range
$80K–$150KBasement/attic conversion (lowest cost)
HUD FY2026Adopted rent benchmark
No state lawRules set city-by-city

ILLINOIS
ADU Collection

Beautiful. Functional. Built for Illinois Living.

🏠Increase Property Value
💲Generate Rental Income
👥Multi-Generational Living
🌿Sustainable Design
Illinois ADU front elevation
Illinois Detached ADU
Illinois Garage Conversion ADU
Illinois Attached ADU
Illinois Basement Conversion ADU
Illinois Modular Prefab ADU
Illinois ADU kitchen interior
Illinois ADU living room interior
Illinois ADU bedroom interior
Illinois ADU bathroom interior
Illinois ADU outdoor living and patio space
Planning & Inspiration
Planning Reference
Example 1-bedroom ADU floor plan, illustrative layout
Example Floor Plan

Illustrative 1-bedroom layout. Actual floor plans vary by ADU type, size, local code, and homeowner needs.

Illinois ADU exterior at twilight, warm interior light glowing
Illinois ADU and main house evening backyard view
Aerial view of Illinois property with backyard ADU
At a Glance

Illinois ADU costs — by type.

Detached ADU
$150K – $350K
Chicago coach houses; $200–$400 per sq ft
🏡
Attached ADU
$100K – $220K
Capped at 700 sq ft or existing unit size in Chicago
🔗
Garage Conversion
$80K – $180K
Built into Chicago's new coach-house category
🚗
Basement Conversion
$80K – $150K
Most affordable; many Chicago bungalows were pre-built for this
⬇️
Modular / Prefab
$100K – $250K
Fastest new-construction timeline
📦
💡

Local control: Local rules control. No broad statewide ADU siting mandate was identified in this review. City, county, zoning, building, utility, and site rules control. Permit fees and review timing remain planning estimates until confirmed for the specific property.

Calculate My ROI →
ADU Types

Which type fits your property?

01 / Chicago Coach House
Detached ADU
$150K – $350K

Chicago's classic coach house typology, once banned for nearly 70 years, is now legal citywide as of April 1, 2026 — capped at 1,000 sq ft and 22 feet in height, sited behind the primary residence.

Detached ADU Guide →
02 / Above-Garage Option
Garage Conversion
$80K – $180K

Chicago's coach house category commonly takes the form of a unit above or beside a garage — a classic Chicago typology that predates the 1957 ban.

Garage Conversion Guide →
03 / Fastest Build
Modular / Prefab
$100K – $250K

Factory-built construction offers a faster, more predictable timeline, with less exposure to Illinois's harsh winter construction season and Chicago's 4-month plan review.

Prefab ADU Overview →
04 / Smaller Lots
Attached ADU
$100K – $220K

Under Chicago's ordinance, a property can add one coach house OR one conversion unit — not both. Attached additions share one wall to reduce site-work cost on tighter city lots.

Attached ADU Guide →
05 / Most Affordable
Basement Conversion
$80K – $150K

The city's most affordable ADU category. Many Chicago bungalows were literally built with basements suited to eventual conversion — the city calls them "pre-ADUs."

Basement ADU Guide →
Cost by Region

Every Illinois market is different.

Chicago — Coach Houses
Logan Square · Lincoln Square
$150K–$350K
$200–$400 per sq ft
Highest cost
Chicago — Conversions
Citywide, 20+ year old buildings
$80K–$150K
Basement / attic units
Moderate
Springfield / Peoria
Downstate Illinois
$85K–$180K
$130–$225 per sq ft
More affordable
Cost Factors

Why Illinois ADUs cost what they cost.

🏛️
Chicago's 2025 ADU Ordinance
After a 46-0 City Council vote, Chicago's permanent citywide ADU ordinance takes effect April 1, 2026, expanding eligibility from about 12% to roughly 60% of the city's residential land area.
Effective April 1, 2026
🏘️
Aldermanic Ward Control
Each alderperson can set their own restrictions for RS zones in their ward — owner-occupancy, permit caps per block, and administrative adjustments can vary block to block.
Rules can differ by block
🏗️
Licensed Architect Requirement
Chicago requires an Illinois-licensed architect for all ADU plans, adding a design-fee floor that doesn't exist in many other states' ADU programs.
Required for all Chicago ADU plans
⚖️
Current ADU Legal Framework
No broad statewide ADU siting mandate was identified in this review. City, county, zoning, building, utility, and site rules control.
Local control · Local rules control
Homeowners reviewing ADU permit documents
Permits & Fees

What permitting actually costs.

No broad statewide ADU siting mandate was identified in this review. City, county, zoning, building, utility, and site rules control. The detailed figures below are representative planning ranges, not a statewide fee schedule or review-time guarantee.

📄
Building Permit Fee — Chicago
$1,500 – $5,000
Through the Chicago Department of Buildings; varies by project scope.
🏗️
Administrative Adjustment (if required)
$500
Required in some wards; involves documentation showing zoning-code compliance.
💧
Utility Connection Fees
$15,000 – $40,000
Separate utility connections for new Chicago coach houses.
🧾
Building Permit Fee — Other Cities
$500 – $2,500
Springfield, Peoria, and other Illinois cities vary widely.
⏱️
Permit Review Timeline — Chicago
~4 months
Plan review through the Department of Buildings currently averages about 4 months.
Verified August 5, 2026 · Local control
Local rules control

No broad statewide ADU siting mandate was identified in this review. City, county, zoning, building, utility, and site rules control.

Legal classificationLocal control
Governing frameworkLocal rules control
Representative permit fees$500 – $2,500
Representative review time~4 months
Local verificationRequired

Source: NationwideADU 51-jurisdiction law directory →

Verify the property before design

These are planning benchmarks, not a permit quote, eligibility ruling, or legal opinion. Confirm zoning, building, utility, site, private-covenant, fee, and timing requirements with the current local authorities. See the 51-jurisdiction law directory and national permit guide.

Return on Investment

Illinois ROI planning inputs.

$775–$1,450
Adopted statewide 1-bedroom rent range
15%
National property-value planning input
10%–20%
Property-value uncertainty range
30%
Operating-expense planning reserve
Rent source
HUD FY2026 revised
One-bedroom FMR market basket
Exact HUD mappings
ROI-CM-1.4
Home-value source
Statewide Zillow ZHVI
June 30, 2026
One statewide input
Not a valuation
Payback period
Calculated live
Budget, ADU type, size and rent
Open calculator →
No static statewide promise
Full methodology
Transparent sources
HUD, Census PUMS, Zillow and FHFA
Review methodology →
Planning estimate, not a valuation
Save Money

7 ways to cut your
Illinois ADU cost now.

Illinois is already one of the more affordable states to build in — these moves push savings further and prevent the most common costly mistakes.

Calculate My ADU ROI →
Compact detached ADU exterior
🏡
Compact Detached ADU
Illinois — statewide range
Property-Value Planning Input
15%
1
Convert your basement instead of building new
Chicago's basement and attic conversion units cost roughly half of a new detached coach house — many Chicago bungalows were literally built with conversion-ready basements the city calls "pre-ADUs."
Saves roughly 50% vs. new coach house
2
Use Cook County's Home Improvement Exemption
A portion of an ADU's added value can be exempted from reassessment for up to four years, softening the property tax increase that typically follows a permitted addition.
Defers tax increase for up to 4 years
3
Check your ward's specific rules before design
Each Chicago alderperson can set additional restrictions for RS zones — rules can differ block to block. A quick check via Chicago Cityscape before design work begins prevents costly redesigns.
Prevents wasted design fees
4
Ask about Chicago's low-income rental subsidies
The Chicago Low Income Housing Trust Fund offers subsidies to owners who rent ADUs to qualifying low-income tenants — worth exploring if your rental strategy fits.
Potential rental subsidy income
5
Get a minimum of three contractor bids
Contractor pricing for ADU projects in Chicago varies significantly for equivalent scope. Getting three bids from ADU-experienced, Illinois-licensed-architect-backed contractors is the most reliable way to avoid overpaying.
Prevents overpaying for the same scope
6
Budget separate utility connections into your total
New coach houses require independent utility connections, which alone can add $15,000–$40,000 to a Chicago project — budget for this early rather than as a late surprise.
Prevents late-stage budget surprises
7
Start design during Chicago's ~4-month review window
Begin contractor and utility-connection planning during the plan review period — not after approval — since Chicago's process currently averages about 4 months.
Cuts months off total project timeline
Frequently Asked Questions

Illinois ADU answers.

Are ADUs legal in Chicago?
Yes. After banning coach houses outright in 1957, Chicago's City Council voted 46-0 in September 2025 to legalize ADUs citywide. The ordinance takes effect April 1, 2026, expanding eligibility from about 12% to roughly 60% of the city's residential land area.
How much does an ADU cost in Illinois?
In Chicago, total ADU project costs typically run $80,000 to $300,000+ depending on unit type. Basement and attic conversions tend to run $80,000–$150,000, while detached coach houses can reach $200,000–$350,000+ once utility connections and finishes are included.
Does Illinois have a statewide ADU law?
NationwideADU's August 5, 2026 review classifies Illinois as local control; no broad statewide ADU siting mandate was identified. No broad statewide ADU siting mandate was identified in this review. City, county, zoning, building, utility, and site rules control. Local zoning, building, utility, site, and private-covenant requirements may still apply; verify the current rules for the property before design or construction.
How long does it take to get an ADU permit in Chicago?
The representative ADU permit-review planning timeline for Illinois is ~4 months. This is not a statewide deadline or guarantee. Timing can change with the local authority, project complexity, corrections, utilities, site conditions, and application completeness; confirm the current timeline locally.
Can I build both a coach house and a basement conversion in Chicago?
No. Under Chicago's ordinance, a property with 1–4 existing units can add one coach house OR one conversion unit — not both. Properties with 5+ units can't add coach houses but may add conversion units up to 33% of existing legal units.
What is the cheapest ADU to build in Illinois?
Basement and attic conversion units in Chicago are generally the least expensive option, often $80,000–$150,000, since the building shell already exists.
What is the ROI on an ADU in Illinois?
The adopted Illinois one-bedroom planning range is $775–$1,450 per month, derived from the HUD FY2026 revised FMR market basket under ROI-CM-1.4. Payback is calculated from the homeowner's budget, ADU type, size, rent range, and a 30% operating-expense reserve. Property value uses a 15% national planning input with a 10%–20% uncertainty range; it is not a valuation.

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Planning Disclaimer: Cost, fee, and timeline ranges are approximations for education and early planning; actual requirements and costs vary with the property, local authority, utilities, site conditions, design, and contractor. The Illinois legal classification and framework shown here were verified on August 5, 2026, but they do not determine parcel eligibility and are not legal advice. Confirm current zoning, building, utility, private-covenant, fee, and timing requirements directly with the applicable authorities before design or construction. Obtain at least three contractor bids before committing to a project budget. Last updated August 2026.