Compare ADU zoning rules across 3 Alabama cities. Average ADU score: 3.0/10.
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 Alabama cities covered here, taken from each city's own ordinance rather than a state summary.
| Question | Across Alabama |
|---|---|
| Are ADUs permitted? | 0 of 3 permit them, 2 only by discretionary approval, and 1 do not. |
| Detached size cap | Median 1,000 sq ft, from 800 to 1,000 sq ft across the 2 cities publishing a numeric cap. |
| Owner-occupancy | Required in 1 of 3. |
| Build cost | $90,000 to $185,000 depending on city and unit type. |
| Easiest and hardest here | Birmingham scores 4/10; Huntsville 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 |
|---|---|---|---|---|---|---|
| Birmingham | No fixed cap for a permitted accessory dwelling. An ADU of 800 sq ft or the size of the primary dwelling (whichever is less) that meets design standards may cover more than 30% of the required rear yard (Ch. 3, Art. I, Sec. 4.D.32); a larger unit must stay within the 30%-of-required-rear-yard limit for accessory buildings (Sec. 6.A) and the district setbacks. | Accessory buildings may stand in the required rear yard but no closer than 3 ft to the rear lot line, or 5 ft where a garage door opens onto an alley; accessory buildings of 700 sq ft or less get half the district rear setback (min. 3 ft) (Ch. 3, Art. I, Sec. 4.D.5 and Sec. 6.A). District rear setbacks: D-3, D-4, D-5 15 ft; D-2 30 ft; D-1 40 ft (Table 1.03.101). | Building permit $9.50 per $1,000 of valuation, $125 minimum, plus a $150 Certificate of Occupancy (2024 Technical Code, Art. V); trades covered under the general contractor pay a processing fee only. A $120,000 ADU is about $1,140 plus the CO. | $90,000-$180,000 | $1,000-$1,300/mo | 4/10 |
| Huntsville | N/A: a detached accessory structure cannot be a dwelling (Sec. 73.5), may hold no plumbing beyond one laundry sink and/or one toilet (Sec. 3.1 'Accessory Structure'; a pool bath house up to 180 sq ft is the only exception) and may not contain a kitchen (Sec. 10.6.3). Accessory structures may cover at most 30% of the required rear yard and rise no more than 2 stories (Sec. 73.8.1). | N/A for ADUs (rear dwellings prohibited, Sec. 73.5). A non-dwelling accessory structure must sit at least 5 ft from all lot lines and 10 ft from any other structure on the lot (Sec. 73.8.1). Main-building rear yard: 45 ft in Residence 1 (Sec. 10.2.5); 25 ft for a two-family dwelling in Residence 2 (Sec. 13.2.2). | Building permit = total contracting price x 0.0055 (0.55%): about $330 on a $60,000 in-law-suite remodel, $825 on a $150,000 addition (City 'Permit Costs' page, read Sept 2026). Trade permits are billed separately. | $95,000-$185,000 | $1,050-$1,150/mo | 2/10 |
| Mobile | No ADU size cap is codified (the 1,000 sq ft / 50% draft rule was not adopted). A second unit must fit the district lot coverage: R-1 Urban 50%, R-1 Suburban 35%, R-2 Urban 50%, R-2 Suburban 40%, R-A 35% (Secs. 64-2-4 to 64-2-6) | District rear yard applies to any dwelling: R-1 Urban 5 ft, R-1 Suburban 8 ft, R-2 Urban 5 ft, R-2 Suburban 8 ft, R-A 10 ft (Secs. 64-2-4 to 64-2-6); Historic District Overlay (Art. 14) can override. No accessory-dwelling setback reduction is codified | Ordinance 28-052 (eff. Jan. 2023): new construction pays $5.50 per $1,000 of valuation to $50,000, then $4.50 per $1,000, with valuation set at $86/sq ft for 1-2 family work; an 800 sq ft unit values at $68,800 for a permit fee of about $360. First residential plan review is free. | $90,000-$175,000 | $750-$1,100/mo | 3/10 |
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