Skip to main content
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)
10–16 yrsTypical payback period
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
📦
💡

No statewide ADU law — rules vary by city. Illinois has no statewide ADU statute. Chicago banned coach houses outright from 1957 until a 2020 pilot program, then went citywide via a 46-0 council vote in September 2025 — effective April 1, 2026. Springfield and Peoria each set their own separate, more limited rules.

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
📋
City-by-City Rules
No statewide ADU law. Chicago's new ordinance is far more developed than most Illinois cities — Springfield, Peoria, and most counties have limited or no formal ADU framework.
Varies entirely by city
Homeowners reviewing ADU permit documents
Permits & Fees

What permitting actually costs.

Illinois has no statewide ADU permit standard. Each city sets its own fees, calculated on project valuation.

📄
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.
Illinois Law
There's no state-level protection here.

Illinois has no statewide ADU statute. Chicago legalized ADUs citywide via a 46-0 City Council vote in September 2025, effective April 1, 2026 — but every other Illinois city and county sets its own separate, often far less developed rules.

Statewide ADU lawNone
Chicago ordinance effectiveApril 1, 2026
Chicago coach house cap1,000 sq ft, 22 ft height
Owner-occupancy requirementYes, in Chicago (2 units or fewer)
Permit review timeline~4 months in Chicago
Short-term rentalsNot allowed — long-term only
💡 Check your ward's specific rules before assuming eligibility

Chicago's ordinance allows each alderperson to set additional restrictions for RS zones in their ward — owner-occupancy rules, permit caps per block, and whether an administrative adjustment is required can all differ block to block. Check chicago.gov/adu or Chicago Cityscape for your specific address before designing.

Return on Investment

What a Illinois ADU returns.

$1,300–$1,600
Monthly rent, conversion units
10–16 yrs
Typical payback period
Up to 4-yr tax exemption
Cook County assessment benefit
~4 months
Chicago permit review timeline
Chicago — Conversion Units
$1,300 – $1,600/mo
Basement/attic monthly rent
10–13 years
Estimated payback
Chicago — Coach Houses
$1,500 – $2,200/mo
1-2 bed monthly rent
12–16 years
Estimated payback
Springfield / Peoria
$900 – $1,300/mo
1-bed monthly rent
10–14 years
Estimated payback
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
Added Property Value
Up to 4-yr Cook County exemption
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?
No. Illinois has no statewide ADU statute — regulation is entirely local. Chicago's new citywide ordinance is far more developed than most of the state; Springfield, Peoria, and most Illinois counties have limited or unclear ADU frameworks.
How long does it take to get an ADU permit in Chicago?
Plan review through the Chicago Department of Buildings currently averages about 4 months. Some wards require an additional administrative adjustment ($500 fee) before permit applications can be submitted.
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 Chicago?
Chicago conversion units renting for $1,300–$1,600/month against an $80,000–$150,000 build cost can produce strong gross yields relative to comparable acquisition-based investments. New coach houses cost more but still generate meaningful rental income at $1,500–$2,200/month in many neighborhoods.

Ready to find out what your ADU will actually cost?

Start with the ROI calculator. Free, no signup, no contractor pitch. Just the data.

Calculate My ROI → All 50 State Guides
Cost Disclaimer: All cost ranges and estimates are approximations based on industry contractor data, published reports, and regional market research as of 2026. Actual ADU costs vary significantly based on lot conditions, city requirements, contractor selection, material specifications, and design complexity. Illinois has no statewide ADU law — rules vary entirely by city, and Chicago's new ordinance took effect April 1, 2026 with ward-level variation still possible. Always verify current zoning and permit requirements directly with your local authority before beginning design. Always obtain a minimum of three contractor bids before committing to a project budget. Last updated June 2026.