Quick answer: A basement apartment conversion in Chicago usually costs about $90,000 to $175,000 for a basement with good ceiling height, and $175,000 to $300,000 or more when the floor has to be lowered or the foundation underpinned. Since April 1, 2026, Chicago’s expanded ADU ordinance allows basement and attic conversion units on far more lots, but each unit still needs city pre-certification, architect-drawn plans, a building permit, legal egress, a fire-rated separation and safe utilities. AMG Proline gives a fixed written price after a free in-home visit. Call (708) 979-3298 or request a free estimate!
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Thousands of Chicago two-flats, three-flats and bungalows have basements that could be homes. For years, most of them could not legally become apartments. That changed when the city expanded its Additional Dwelling Unit rules. Now many owners can add a legal garden unit for rental income, family members or aging parents. The opportunity is real, but so is the work. A legal unit has to meet the building code for light, air, ceiling height, fire safety and emergency escape, and basements bring water, sewer and structural challenges that above-grade units do not. This guide walks through the rules, the construction and the real costs.
This guide goes with our basement remodeling in Chicago service. For finished basements that are not separate units, see Basement Remodel Cost in Chicago.
Wondering if your basement can become a legal unit? We check ceiling height, egress, plumbing and water issues before we price anything. Call (708) 979-3298!
Chicago ADU Rules for Basement Conversion Units
Chicago calls these units Additional Dwelling Units, or ADUs. A basement or attic apartment inside an existing building is a conversion unit. A separate building in the backyard is a coach house. The rules below cover conversion units. They apply inside the City of Chicago only. Suburbs set their own rules.
| Rule | What it means for a basement unit |
|---|---|
| Effective date | The expanded citywide ordinance took effect April 1, 2026, building on the 2021 pilot program |
| Where it applies | By right in many multi-unit residential districts and some business zones. Single-family zones are included only in wards that opted in, with rules that vary by area |
| Building age | The building must have been residential for at least 20 years |
| How many units | Buildings with one to four units can add one conversion unit. Larger buildings can add more, capped at 33 percent of existing units |
| Affordability | When two or more ADUs are added to a property, every other unit must be affordable to households at or below 60 percent of area median income for 30 years |
| Local limits | Some single-family areas have annual caps on pre-certified units or owner-occupancy rules |
| Short-term rentals | ADUs cannot be used as short-term rentals such as Airbnb or Vrbo |
How the approval process works
- Check eligibility using the city’s ADU address lookup.
- Apply for pre-certification with the Department of Housing, with basic details such as ownership, building age and unit count.
- Hire an architect or engineer to prepare code-compliant plans.
- Apply for a building permit with your pre-certification letter.
- Build and pass inspections before anyone moves in.
Rules can change, and eligibility depends on your exact address. Confirm with the city before you design. For how permits work across the area, see our room-by-room permit guide.
Is Your Basement a Good Candidate?
| Factor | Good sign | Expensive sign |
|---|---|---|
| Ceiling height | Close to code height after the finished floor and ceiling | Low ceilings that need the floor lowered |
| Water history | Dry basement, working drain tile and sump | Past flooding, seepage or sewer backups |
| Windows | Existing large windows or room to enlarge them | Small windows below grade with no room for wells |
| Entrance | Existing rear or side door to the outside | No exterior door location |
| Plumbing | Existing drains at or below the basement floor | Fixtures that need an ejector pit |
| Electrical and heating | Room for a new subpanel and a separate heating system | Old service with no capacity |
Ceiling Height and Lowering the Floor
Ceiling height is the first thing we measure. Building codes set a minimum clear height for habitable rooms, commonly around 7 feet, with limited allowances for beams and ducts. Your architect confirms the exact requirement for your building.
Many older Chicago basements fall short once you add a new floor, insulation and drywall. There are three ways to solve it:
- Reroute ducts and pipes into soffits along the walls so the main ceiling stays high.
- Lower the floor by removing the slab, digging down and pouring a new slab. This is often called a dig-out.
- Underpin the foundation when digging below the existing footings. Underpinning extends the foundation down in sections and needs an engineer’s design.
Lowering the floor and underpinning are the biggest single costs in many conversions, but they also add real value because the finished unit feels like a full apartment instead of a basement.
Egress and Exits
Every sleeping room in a basement unit needs a way out in an emergency, and the unit needs a proper exit to the outside.
Emergency escape windows
- Size. Code-compliant egress windows need a minimum clear opening area, width and height, and the sill can only sit so high above the floor. Common values are about 5.7 square feet of opening, with at least 20 inches of width and 24 inches of height, and a sill no higher than 44 inches.
- Window wells. A below-grade egress window needs a well large enough to climb out of, with a ladder or steps when the well is deep.
- Foundation cutting. Enlarging a window means cutting the foundation wall and adding a new header and frame.
- Drainage. Every new well needs a drain so it does not fill with water.
Entrances
Most basement units need their own entrance to the outside, often through an existing rear door or a new exterior stair. The path out has to meet code for stair size, headroom and lighting. Shared interior stairs with the main building usually need fire-rated doors.
Fire Separation and Safety
- Fire-rated separation. The ceiling and walls between the basement unit and the rest of the building usually need a fire-resistance rating, typically with fire-rated drywall and sealed penetrations.
- Fire-rated doors where the unit connects to shared spaces.
- Smoke and carbon monoxide alarms in the locations the code requires, usually hardwired and interconnected.
- Separated mechanical rooms so a furnace or water heater fire does not spread into living space.
- Sound control. Insulation and resilient ceiling details make life better for tenants above and below.
Utilities: Electrical, Heating, Plumbing and Sewer
Electrical
A new unit usually needs its own panel or subpanel, and often its own meter. Older Chicago buildings may need a service upgrade to carry a full kitchen, laundry and heating load. Chicago also requires wiring to run in conduit, which adds labor to every circuit.
Heating and cooling
Many conversions use a separate furnace, a heat pump or a ductless mini-split system for the new unit. Separate systems give tenants control, avoid shared ductwork between units and simplify billing.
Plumbing and sewer
- Kitchen and bathroom drains. Fixtures below the sewer line need an ejector pit to pump waste up. See Basement Bathroom Ejector Pit Cost in Chicago.
- Flood protection. Chicago basements are prone to sewer backups in heavy storms. An overhead sewer or backflow protection is one of the best investments for any garden unit.
- Water heater capacity. A separate water heater for the unit is common.
- Laundry. In-unit or shared laundry affects both layout and drain planning.
Waterproofing
A basement unit is only as good as the foundation around it. Drain tile, sump pumps, wall waterproofing and vapor control come before any finishes. Read Why We Won’t Cut Corners on Waterproofing and choose floors that handle moisture with our guide to the best flooring for basements.
Not sure what your basement needs? We look at water, sewer, ceiling height and egress in one visit.
Real Costs: What a Basement Apartment Conversion Costs in Chicago
These are planning ranges based on basement projects across Chicago. Your fixed price depends on what the inspection finds and what your plans require.
| Project type | What’s included | Planning range |
|---|---|---|
| Studio or one-bedroom, good ceiling height | Egress windows, kitchen, bathroom, fire separation, new electrical and HVAC, finishes | $90,000 to $175,000 |
| Two-bedroom unit | More egress windows, larger kitchen and bath, laundry | $130,000 to $220,000 |
| Conversion with floor lowering or underpinning | Dig-out, new slab, engineered underpinning, plus the full unit | $175,000 to $300,000+ |
Where the money goes
| Line item | Typical range |
|---|---|
| Architect plans and engineering | $8,000 to $25,000 |
| Floor lowering and underpinning | $40,000 to $120,000+ when needed |
| Egress windows and wells | $6,000 to $15,000 each |
| New exterior entrance or stair | $10,000 to $35,000 |
| Waterproofing, drain tile and sump | $10,000 to $35,000 |
| Overhead sewer or backflow protection | $8,000 to $25,000 |
| Electrical service, panel and wiring | $12,000 to $30,000 |
| HVAC for the unit | $8,000 to $20,000 |
| Kitchen | $15,000 to $40,000 |
| Bathroom with ejector pit | $18,000 to $40,000 |
| Fire separation, insulation and drywall | $15,000 to $35,000 |
| Flooring, doors, trim and paint | $12,000 to $30,000 |
| Permits and inspections | Set by the city based on scope |
We price every line before work begins, so you are not surprised after demolition. Read Why We Quote Fixed Written Prices and In-House Trades vs. Subcontractors.
Hidden costs in older buildings
Chicago basements often hold surprises once walls and floors open: old drain lines, outdated wiring, asbestos pipe insulation or lead paint on older surfaces. See What Happens When Asbestos Is Found During a Remodel, Lead Paint and Pre-1978 Homes and What Happens When You Open a Wall in an Older Chicago Home.
Is a Basement Apartment Worth It?
- Rental income can help pay down the project over time.
- Multi-generation living gives parents, adult children or caregivers a private home on the same property.
- Property value can rise with a legal, permitted unit, compared with an unpermitted one.
- Flexibility to rent now and use the space for family later.
It may not be worth it if your basement has serious water problems you cannot solve, if the ceiling height requires a very costly dig-out compared with the rent you expect, or if your address is not eligible. Owners thinking about the opposite move, combining units, should read Converting a Chicago Two-Flat Into a Single-Family Home.
Typical Timeline
| Stage | Typical time |
|---|---|
| Eligibility check and pre-certification | A few weeks |
| Design, engineering and plans | 1 to 3 months |
| Building permit | Varies with city review and plan complexity |
| Construction, standard conversion | 3 to 5 months |
| Construction with floor lowering or underpinning | 5 to 8 months |
| Final inspections and occupancy | A few weeks |
About AMG Proline
AMG Proline has been remodeling homes in Chicago and the suburbs since 2000. We’re licensed and bonded in Illinois (License #104.020448) and BBB A+ rated. Our own crews handle demolition, framing, waterproofing, tile and finish work, from our showroom at 4711 W 135th St, Crestwood, IL 60418. Learn more about us, see our projects, or read our Google reviews. Before hiring anyone for a project this size, read How to Hire a Licensed Contractor in Illinois.
Basement Apartment Conversion FAQs
Can I legally turn my basement into an apartment in Chicago?
Often yes. Chicago’s expanded ADU ordinance, effective April 1, 2026, allows basement conversion units on many more lots. Eligibility depends on your zoning, ward and building, so check your address with the city first.
How much does a basement apartment conversion cost in Chicago?
Most run about $90,000 to $175,000 with good ceiling height, and $175,000 to $300,000 or more when the floor must be lowered or the foundation underpinned.
What is a conversion unit?
It is Chicago’s term for a new apartment created inside an existing building, usually in the basement or attic.
How many basement units can I add?
Buildings with one to four units can add one conversion unit. Larger buildings can add more, up to 33 percent of the existing units.
How old does my building need to be?
The building must have been residential for at least 20 years to add a conversion unit.
What ceiling height does a basement apartment need?
Habitable rooms need a minimum clear ceiling height set by code, commonly around 7 feet, with limited allowances for beams and ducts. Your architect confirms the exact requirement.
Do basement bedrooms need egress windows?
Yes. Every sleeping room needs an emergency escape opening that meets code for size and sill height, with a window well and drain when it is below grade.
Does a basement unit need separate utilities?
Most conversions add a separate electrical panel, heating system and water heater, and often separate meters. Your plans set what is required for your building.
Can I rent my basement unit on Airbnb?
No. Chicago ADUs cannot be used as short-term rentals.
Do I need an architect?
Yes. The city requires code-compliant plans prepared by an architect or engineer for the building permit.
How long does a basement apartment conversion take?
Design and approvals often take 2 to 4 months, and construction about 3 to 5 months, or 5 to 8 months with floor lowering or underpinning.
How do I get a price?
Call (708) 979-3298 or request a free estimate online. We visit, measure and give you a fixed written price.
Get Your Free Basement Apartment Estimate
Call (708) 979-3298 · 4711 W 135th St, Crestwood, IL 60418 · Monday to Saturday, 8 AM to 7 PM
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