Ordinance explainer · Additional Dwelling Units
ADUs and coach houses in Chicago: where a second unit is actually legal
Chicago's zoning ordinance recognizes two kinds of additional dwelling unit: the conversion unit (a new unit carved out inside an existing building — attic, basement, subdivision) and the coach house (a separate rear building). They are permitted by right, citywide, in RT and RM districts and most B and C districts (§17-2-0207, §17-3-0207). In the single-family RS-1, RS-2 and RS-3 districts they are legal only inside ten designated ADU-Allowed RS Areas (§17-7-0570) — several of which add owner-occupancy requirements and annual per-block caps. C3, downtown and manufacturing districts don't allow them at all.
Ordinance text quoted from Municipal Code edition 2026-S-73 (current through 2026-02-18); the ADU sections were amended as recently as Feb 18, 2026. Data snapshot generated 2026-08-24. How this data is produced and checked →
1 · Where each ADU type is allowed
| District family | Conversion unit | Coach house | Source |
|---|---|---|---|
| RS-1 / RS-2 / RS-3 | By right only within an ADU-Allowed RS Area; otherwise not allowed | §17-2-0207 + §17-7-0570 | |
| RT-3.5, RT-4, RM-4.5 – RM-6.5 | By right | By right | §17-2-0207 |
| B1 / B3 / C1 / C2 | Permitted; special-use approval in some cases | By right | §17-3-0207 |
| B2 | By right | By right | §17-3-0207 |
| C3 | Not allowed — no household living in C3 | §17-3-0207 | |
| D (downtown) & M (manufacturing) | No coach-house or conversion-unit entry in the use tables — not available | §17-4-0207, §17-5-0207 | |
2 · The RS pilot: ten ADU-Allowed Areas, three kinds of limits
The ordinance designates ten ADU-Allowed RS Areas across the city (boundaries set street-by-street in §17-7-0574) and explicitly frames them as a cross-section for evaluating the program (§17-7-0571). Three limitation types apply, area by area (§17-7-0573):
- Annual caps. In areas where this applies, pre-certifications per calendar year on the same block face plus the opposite block face are capped at 1 in RS-1, 2 in RS-2, 3 in RS-3 — conversion units and coach houses combined.
- Owner-occupancy. Where the principal building is a detached house, it must be owner-occupied at the time of pre-certification.
- Administrative adjustment. Some areas require approval of an administrative adjustment (§17-13-1003-QQ) before any ADU is authorized.
Three of the ten areas carry no limitations at all; the rest apply one to three of them. Whether a specific lot falls inside an area — and which limits attach — turns on the §17-7-0574 boundary descriptions, which is exactly the kind of parcel-level question the ordinance text alone won't answer quickly.
Process note
ADU approval runs through Department of Housing pre-certification (§17-7-0572, defined in MCC §2-44-106(g)), and coach houses must meet the design and dimensional standards of §17-9-0201-F — both live outside Title 17's bulk tables, so confirm the current standards with DOH and the Zoning Administrator before designing to a number.
3 · The conversion-unit math
For a residential building that has lawfully existed 20 or more years, density may exceed the district's lot-area-per-unit standard by adding conversion units equal to 33% of the lawfully established existing units (conversion units themselves excluded from the count). Fractions of 0.5 and above round up; below 0.5 round down. A building with a single unit may always add one. (§17-2-0303-C for residential districts; §17-3-0309 for B, C1 and C2.)
Worked examples: a 20-year-old six-flat → 33% of 6 = 1.98 → 2 conversion units. A single-family home in an eligible district → 1 conversion unit, regardless of lot size.
4 · Siting, parking, and pre-existing coach houses
- Alley setback. A coach house adjacent to a public alley sits at least 2 ft off the rear property line — waived where the building is at least 10 ft from the alley centerline, evidenced by survey (§17-17-0309). Coach houses may encroach into the required rear setback, not front or side.
- Parking. On a building lawfully in existence 20+ years, parking standards are triggered only when 2 or more units are added (§17-10-0101-B.2(a)) — one ADU on an older building generally adds no parking requirement.
- Existing unpermitted coach houses. Outside landmark districts, continued occupancy is protected unless the unit has been continuously vacant for over a year; repairs yes, expansion no (§17-15-0303-E).
- Related, but not an ADU: in RS-3, on blocks where 60%+ of lots on the same side of the street already hold multi-unit buildings (or inside the §17-7-0591 Predominance-of-the-Block boundaries), the lot-area-per-unit standard drops to 1,500 sq ft — enough to build a two-flat outright (§17-2-0303-B.1).
Chicago ADUs — common questions
- Are ADUs legal citywide in Chicago?
- Not in single-family (RS) zoning. In RT and RM districts, conversion units and coach houses are permitted by right citywide (§17-2-0207). In RS-1, RS-2 and RS-3, they are by right only inside one of ten designated ADU-Allowed RS Areas (§17-7-0570) — the ordinance frames those areas as an evaluation program, and outside them an ADU is not allowed. Most B and C districts also permit them; C3, downtown D and manufacturing M districts do not.
- How many units can a conversion add?
- A residential building lawfully in existence for 20 or more years can add conversion units equal to 33% of its lawfully established units, with fractions of 0.5 and up rounding up — and a single-unit building can always add one (§17-2-0303-C; §17-3-0309 for B/C1/C2 districts).
- Do I need to live on the property?
- In several of the ten ADU-Allowed RS Areas, yes: where the principal building is a detached house, it must be owner-occupied at the time of pre-certification (§17-7-0573(2)). Which areas apply which limitations is set lot-by-lot in §17-7-0574. Outside RS districts, the use tables attach no owner-occupancy condition.
- Does adding an ADU trigger new parking?
- For a residential building lawfully in existence 20+ years, the parking standards apply only when 2 or more dwelling units are added (§17-10-0101-B.2(a)) — so a single ADU on an older building generally triggers no new parking requirement, and existing spaces cannot be reduced below the applicable minimum.
- What about an old coach house that already exists?
- A coach house that lost its legal status can keep being occupied if it has not sat continuously vacant for more than a year (§17-15-0303-E; on designated Landmark properties, the Zoning Administrator instead looks for competent evidence of prior legal dwelling use, §17-15-0303-D). Repairs and maintenance are allowed; expansions are not.