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Mobile, AL Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified September 2026

ADU rules in Mobile, AL, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

Size cap: 50% of main house's floor area
Height cap: no taller than principal home
Location: behind the principal dwelling
Where it applies: Historic Avenue Overlay lots only

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in Alabama under recent ADU streamlining laws. The Mobile, AL rules below cover local size, height, and setback requirements.

Summary

In Mobile's Historic Avenue Overlay, one accessory dwelling unit is allowed by right per single-family lot, capped at 50 percent of the main house's floor area and no taller than the principal dwelling.

(c)Where allowed.(1)ADUs are permitted by right in the overlay on residential lots that do not have more than one (1) dwelling unit. ADUs are not permitted on lots containing a duplex, triplex or a cottage court.(d)Dimensional standards.(1)ADUs shall not exceed a gross floor area of more than 50% of the principal structure's floor area.(2)No ADU may be taller than the principal dwelling unit.(e)Location of ADU.(1)An ADU shall be located behind the principal dwelling and comply with the setback requirements of section E, Dimensional standards, of this overlay.

View official code

Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Code of Ordinances: Supplement 67).

Full Breakdown

Mobile's Historic Avenue Overlay -- roughly the Spring Hill Avenue, Three Mile Creek, and Dr. Martin Luther King Jr. Avenue corridor -- allows one accessory dwelling unit by right on any residential lot of record carrying no more than one dwelling unit; it is not allowed on lots holding a duplex, triplex, or cottage court. Under § 64-15-1(D)(3)(d)-(e), the ADU's gross floor area cannot exceed 50 percent of the principal structure's floor area, it cannot be taller than the main house, and it must sit behind the principal dwelling, clear of any recorded easement, while also meeting the overlay's setback standards.

Violations & Fines

Building an ADU that's oversized, taller than the main house, sited in front of it, or placed on an ineligible lot is a UDC violation subject to stop-work orders and withheld permits.

Frequently Asked Questions

How big can an ADU be in Mobile's Historic Avenue Overlay?

No more than 50 percent of the principal home's gross floor area, and it can't be taller than the main house, per § 64-15-1(D)(3)(d).

Can I add an ADU to a duplex lot?

No. Section 64-15-1(D)(3)(c) permits ADUs only on lots with no more than one existing dwelling unit -- duplexes, triplexes, and cottage courts don't qualify.

Sources & Official References

[1]Mobile Code of Ordinances (Municode)

Other rules in Mobile

Noise Ordinances: Construction HoursSome Restrictions Shed & Carport RulesSome Restrictions Tiny HomesLight Restrictions Parking Rules: Towing & Impound RightsSignificant Restrictions Fence Regulations: Fence Permit RequirementsSome Restrictions Fence Regulations: Neighbor Fence RulesSome Restrictions Animal Ordinances: Veterinary Clinic ZoningSignificant Restrictions Fire Regulations: Smoke DetectorsSome Restrictions

All Mobile rules

How Mobile compares: Easiest Cities to Build an ADU·Compare Mobile to another location·View the Alabama accessory structures overview

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