Zoning & Size Limits
| Requirement | Ann Arbor, MI | Detroit, MI |
|---|
| Allowed Zones | R1, R2, R3, and R4 districts, on lots containing one single- | Currently not permitted in R1/R2; the pending 'Let's Build M |
| Max Size (Detached) | 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 | Proposed: no more than the smaller of 1,200 sq ft or 60% of the principal dwelling's gross floor area (Sec. 50-12-466); not yet in effect |
| Max Size (Attached) | Same tiered cap: 600 sq ft (lots under 7,200 sq ft) or 800 sq ft (lots 7,200 sq ft+), or the main dwelling's ground-floor area, whichever is less | Proposed rules allow only detached ADUs — the unit must be detached from the principal residence (it may be freestanding or combined with a garage, on any story including above the garage) |
| Max Height | 21 ft for an ADU in an accessory building (detache | Proposed: 25 ft maximum for ADUs per the amended d |
| Parking Required | No additional parking required — the parking requirement was | Proposed: no additional parking required, and if the ADU rep |
| Owner Occupancy | Not required — the owner-occupancy requirement was removed b | No owner-occupancy requirement appears in the proposed ordin |
Setback Requirements
| Setback | Ann Arbor, MI | Detroit, MI |
|---|
| Rear Setback | 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 | Proposed: standard 3 ft accessory-structure rear setback, with no rear setback required where the lot abuts an alley; existing accessory structures converted to ADUs may keep their footprint |
| Side Setback | Detached ADU allowed in the side setback area if farther from the street than the main building (and neighbors' main buildings) and at least 3 ft from any lot line; fire-rated walls if 3-5 ft. Attached ADUs may not be in the required side setback | Proposed: 3 ft from side lot lines (accessory-structure standard); ADU must also be at least 10 ft from the principal building, 5 ft from its porch/deck, and 10 ft from any electrical line unless BSEED waives it |
| Front Setback | Not permitted within the required front setback ar | Proposed: ADUs may not be located in a front yard |
Permits & Timeline
| Detail | Ann Arbor, MI | Detroit, MI |
|---|
| Permit Timeline | No hearings required, so review runs on standard building-permit timelines — confirm current plan-review turnaround with Ann Arbor Planning Services (734-794-6265) | Not applicable until the ordinance is adopted — City Council was still deliberating as of spring 2026 |
| Permit Fees | Building and trade permit fees are based on construction valuation — confirm current fee schedule with the Ann Arbor Building Department | Building permit fees based on construction value — confirm with Detroit BSEED once ADU rules take effect |
| Impact Fees | None — Michigan law does not authorize municipal development | None — Michigan law does not authorize municipal development |
| Pre-Approved Plans | No city pre-approved ADU plan library — confirm wi | None for ADUs — confirm with the Detroit Planning |
Construction Costs
| Cost | Ann Arbor, MI | Detroit, MI |
|---|
| Avg Build Cost | $120,000-$240,000 | $90,000-$200,000 |
| Cost Per Sq Ft | $180-$300 | $140-$240 |
| Garage Conversion | $60,000-$130,000 | $45,000-$100,000 |
| Prefab ADU | $90,000-$180,000 | $75,000-$150,000 |
Rental Income Potential
| Rental | Ann Arbor, MI | Detroit, MI |
|---|
| 1BR Avg Rent | $1,450-$1,650/mo | $950-$1,300/mo |
| 2BR Avg Rent | $1,900-$2,000/mo | $1,100-$1,500/mo |
| Short-Term Rentals | No — leasing or renting an ADU for less than 30 days is proh | Proposed ordinance prohibits renting an ADU for 27 or fewer |
| Est. ROI | 6-10% annual return | 8-12% potential annual return once ADUs are legalized — not currently buildable in most districts |
Which City Is Better for ADUs?
Ann Arbor scores 7/10 while Detroit scores 2/10 on our ADU-friendliness index.
Ann Arbor edges ahead overall. Both cities offer viable ADU development opportunities — the best choice depends on your specific property, budget, and goals.
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