Compare ADU zoning rules across 3 Michigan cities. Average ADU score: 5.7/10.
Gathered from the 3 cities below, with the bill or ordinance each one names. Statewide measures are listed once with the cities they reached, rather than repeated for each.
Statewide law sets a floor; the numbers that decide your project are local. Here is the spread across the 3 Michigan cities covered here, taken from each city's own ordinance rather than a state summary.
| Question | Across Michigan |
|---|---|
| Are ADUs permitted? | 2 of 3 permit them, and 1 do not. |
| Detached size cap | Median 850 sq ft, from 600 to 1,200 sq ft across the 3 cities publishing a numeric cap. |
| Owner-occupancy | Required in none of them. |
| Build cost | $90,000 to $240,000 depending on city and unit type. |
| Easiest and hardest here | Grand Rapids scores 8/10; Detroit scores 2/10. |
Recomputed whenever a city page changes, so this and the city pages below cannot disagree.
| City | Max Size | Setbacks | Permit Fees | Build Cost | 1BR Rent | Score |
|---|---|---|---|---|---|---|
| Ann Arbor | 600 sq ft on lots up to 7,199 sq ft; 800 sq ft on lots 7,200 sq ft or larger — or the ground-floor area of the main dwelling, whichever is less | Allowed in the rear setback area if at least 3 ft from any lot line and all structures cover no more than 35% of the rear setback area; fire-rated walls required if 3-5 ft from a lot line | Building and trade permit fees are based on construction valuation — confirm current fee schedule with the Ann Arbor Building Department | $120,000-$240,000 | $1,450-$1,650/mo | 7/10 |
| Detroit | No ADU size rule exists because no ADU use exists. The lapsed proposal (draft Sec. 50-12-466(1)e) would have capped an ADU at the smaller of 1,200 sq ft or 60% of the principal dwelling's gross floor area. Today a building accessory to a one- or two-family house is limited to one story and 15 ft (Sec. 50-13-209). | Garages and other accessory structures: 3 ft from the rear lot line, or up to the lot line where an alley gives vehicle access (Sec. 50-13-226 encroachment table). The lapsed proposal kept 3 ft and would have waived the rear setback for ADUs on lots that abut an alley (draft Sec. 50-13-212(4)). | BSEED building permit (fee schedule modified 7/18/2025): $2,895.29 at $100,000 project cost plus $27.82 per $1,000 above that (about $4,286 at $150,000); trade permits and a 35% plan-review deposit are extra. Relevant only if the zoning changes. | $90,000-$200,000 | $950-$1,250/mo | 2/10 |
| Grand Rapids | 1,000 sq ft gross floor area maximum, and the ADU may never be larger than the primary house (Sec. 5.9.03 as amended June 2026; replaced the 850 sq ft / 40%-of-house cap). Gross floor area is measured from the exterior faces of the structure. Minimum greenspace (LDR 40-60%, MDR 30-50% depending on neighborhood classification) counts all buildings, pavement, decks and patios and can limit the footprint further | Detached ADU: accessory-structure setback of 3 ft from the rear lot line (Sec. 5.9.03(E)(2); June 2, 2026 city briefing). Attached ADU: the primary-structure rear setback of its zone district (25 ft in the Traditional Neighborhood classification). An Administrative Departure ($260) can allow a detached unit as close as 1 ft where site constraints justify it | FY2027 residential chart (from July 1, 2026): $54 application + $6.80 per $1,000 of construction value above the first $1,000 + $25 zoning permit, so $752.20 at $100,000, $1,092.20 at $150,000 and $1,432.20 at $200,000; trade permits and water/sewer connection fees are extra. | $90,000-$200,000 | $1,280-$1,470/mo | 8/10 |
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