Adding a Basement or Garden Apartment in Chicago: The ADU Rules Explained

01

What Chicago Means by a Conversion Unit

Chicago’s code separates ADUs into 2 types, as the city’s ADU program page explains. A conversion unit is connected to the existing building, such as an apartment in an attic or basement. A coach house is a separate, detached structure. What many Chicagoans call a garden apartment is, in the city’s terms, a basement conversion unit.

These units are not new to the city. The same page notes that ADUs were common in Chicago through the first half of the 20th century, until zoning rules starting in 1957 added parking requirements and banned secondary residential structures.

City Council first approved ADUs in 2020, in 5 pilot areas. On September 25, 2025, it passed ordinance SO2024-0008918 to expand ADUs beyond the pilot, and the ordinance took effect on April 1, 2026. At passage, the Mayor’s office said the expansion would more than double the number of ADU-eligible parcels, to 245,000 from about 116,000 under the pilot.

02

Where Basement Apartments Are Allowed in 2026

As of 2026, the city allows ADUs by right in all multi-unit residential, mixed-use and certain business and commercial zoning districts, according to its eligibility and requirements pages. Business and commercial districts can add rules about where in the building a conversion unit may sit, so check the city’s chart for your district.

Single-family zoning works differently. Properties zoned RS-1, RS-2 or RS-3 can add an ADU only inside an “ADU-Allowed RS Area.” These areas are created and changed by City Council ordinance, so the map can grow over time. Under the 2025 ordinance, the 5 pilot zones (North, Northwest, West, South and Southeast) became the first of these areas.

The quickest first step is the city’s eligibility map, linked from the eligibility page, which shows whether your address can add an ADU and which limits apply. The City of Chicago Zoning Map shows your exact district.

Limits That Can Apply in Single-Family Areas

An ADU-Allowed RS Area can carry extra limits. The city lists 3:

• Annual block caps. Only so many conversion units and coach houses, combined, can be pre-certified on the same block face and the block face across the street in a calendar year: 1 in RS-1, 2 in RS-2 and 3 in RS-3.
• Owner occupancy. Where this limit applies and the main building is a detached house, the owner must live there to receive pre-certification. Proof is a government-issued photo ID plus 1 document such as a recent property tax bill, a current utility bill, a recorded deed, a mortgage statement or a homeowner’s insurance declaration.
• Administrative adjustment. Where required, the applicant must obtain an administrative adjustment through the Department of Planning and Development, including a $500 fee.

In the ordinance as passed, the West, South and Southeast zones carry the block cap and the owner-occupancy limit, while the North and Northwest zones list neither. Areas added later can carry their own limits, so read what the map shows for your address.

03

How Many Units Your Building Can Add

For a conversion unit, the main building must be at least 20 years old. The number you can add is based on the existing legal dwelling units, at what the city calls a 33% additional density rate. The city’s table, as of 2026:

So a single-family home, a two-flat or a 4-unit building can add 1 basement or attic unit, if its zoning and area allow it. The city also allows 1 coach house, alone or together with conversion units. If a coach house is on your mind too, see our guide to converting a garage to a legal coach house and our page on garage remodeling in Chicago.

3 more rules travel with every conversion unit:

• No short-term rentals. Short-term leases and vacation rentals, such as Airbnb, are not permitted. The ordinance sets a fine of $1,500 to $3,000 per offense, with each day counted as a separate offense.
• No added parking. The city states that no additional parking is required.
• Affordability once you add 2 or more. When an owner adds 2 or more ADUs, in any mix of conversion units and coach houses, 50% of the new units (rounded down) must stay affordable for 30 years, rented to tenants at or below 60% of Area Median Income. Per the city’s affordability page, the owner picks which units, registers each one with the Department of Housing for a $500 fee, records a covenant with the Cook County Clerk and files an affidavit every year. A single basement unit on its own does not trigger this rule.

04

Pre-Certification Comes Before the Permit

The city’s ADU process runs in a fixed order. Following it means you pay for full permit drawings only after you know the address qualifies.

Step 1: Department of Housing Pre-Certification

You apply online to the Department of Housing (DOH). The application asks for applicant details, the property address, ownership and occupancy information, the number of existing legal units, when the building was built, and the type and number of ADUs. Owner-occupancy proof and affordability documents are uploaded only if they apply.

When you submit, the system automatically emails your alderperson’s office with the applicant’s name, contact information and the project address. DOH staff review the application and notify you of approval or denial by email. An approval comes as a pre-certification letter.

Step 2: Design by a Licensed Professional

Next, a licensed architect or structural engineer designs the unit and prepares the plans for the building permit application, making sure the project meets Chicago’s building codes and zoning requirements.

Step 3: Department of Buildings Permit

You then apply to the Department of Buildings (DOB) for a plan-based building permit, submitting the plans with the pre-certification letter. Expect review comments: the architect may need to revise the plans to meet code. Once the permit is approved, you pay the permit fee.

The city is clear on one point. A pre-certification letter does not guarantee a permit, and no construction can begin until DOB issues it.

Step 4: Construction, Inspections and Occupancy

Licensed contractors and subcontractors build the unit, with in-progress and final inspections as the work advances. The city says a Certificate of Occupancy is required for buildings of 4 or more units, and that for 1 to 3 units, or a coach house, you may request one before inspections. If affordability rules apply, annual affidavits follow.

The codes themselves are also changing. Under the city’s Phase 3 code update, applicants may elect amended Building Code and Building Rehabilitation Code provisions as of October 1, 2026, and the new provisions are mandatory for permit applications started on or after December 1, 2026. If you apply around that date, ask your architect which version your plans follow.

05

What a Basement Unit Has to Get Right

Zoning says a unit is allowed. The building code decides whether your basement can actually become one. These are the same questions that shape any basement remodeling in Chicago, with higher stakes for a space someone will live in full time.

The city’s ADU resources page points owners to Better Basement Units, a manual by Neighborhood Housing Services of Chicago written during the pilot program. Its checklist is a useful way to see the issues early. Exact dimensions come from the current code and your architect, not from a rule of thumb.

Ceiling Height

The manual calls height a common limitation for basement units. Measure the clear height under the lowest beam, duct or pipe, not only between joists, and leave room for the new floor and ceiling layers. Lowering the floor is possible, but the manual describes excavating around the foundation as expensive and risky, so the existing height often shapes the whole plan.

Light, Air and Water

Each room needs enough natural light and ventilation, which the code ties to window area relative to floor area. Basements often have small windows, so new or larger openings or exterior light wells may be needed, and those can run into structure and property lines. The manual also calls for mechanical ventilation in new bathrooms, kitchens and laundry rooms.

Water is the other half. The manual notes that a dry basement depends on soils, elevation and groundwater control. Drainage from downspouts, areaway drains and foundation drains needs a sump pump, and if your sewer line runs overhead, new fixtures need an ejector pump to reach it. Settle waterproofing before finishes.

Exits and Fire Safety

A basement unit needs a safe way out: a direct exit from the unit, or exits through common hallways within the travel distances the code sets. The manual also calls for fire-rated partitions between units and around common exit paths, interconnected smoke alarms, and carbon monoxide alarms near fuel-burning appliances such as furnaces, water heaters and gas stoves. Rated walls and ceilings take up thickness, which brings the conversation back to height.

Separate Systems

The manual notes that a multi-unit building needs a separate metered electric service for each unit and one for common areas. Heat, plumbing and sewer capacity need checking too. Older buildings with shared steam heat may need a supplemental system for the new unit rather than changes to the floors above.

06

How a Design-Build Team Plans a Basement Apartment

Because the city steps and the building facts depend on each other, the order of work matters. A sound sequence looks like this:

1. Confirm eligibility first. Check the address on the map, the zoning district, the age of the building and the legal unit count before paying for full drawings.
2. Measure the basement honestly. Record height under every obstruction, window sizes, possible exit routes, sewer elevation, utility capacity and any history of water.
3. Design around the hard limits. Lay out rooms around height, light, exits and fire separation, then choose finishes.
4. Follow the city order. DOH pre-certification, then permit plans from the licensed architect or engineer, then the DOB permit.
5. Build, inspect and close out. Settle the Certificate of Occupancy question before inspections begin, then schedule inspections as the work advances.

At Areté, design and construction have sat under one roof since 2002, and our in-house interior design team helps choose finishes that hold up in a home someone else will live in, whether a tenant or a parent. Each project is run by a Project Manager, from the first measurement to the final inspection. For design ideas for the space itself, browse our Chicago basement remodel ideas for 2026, and see how we approach below-grade work on our page about basement remodeling in Chicago.

07

Frequently Asked Questions