Everything homeowners ask about building an ADU in Mobile — answered with 2026 data.
Sometimes — but not by right. The adopted UDC (Ord. 64-017-2022) lists no Accessory Dwelling Unit in its Use Table and caps R-A and R-1 lots at one dwelling, so outside downtown an ADU needs a Planning Director similar-use determination or a Board of Zoning Adjustment variance. R-2 lots can add a second unit by right as a two-family dwelling, and the Downtown Development District allows ADUs by right in T-3, T-4, T-5.1 and SD-WH. Zoning: Not listed in the UDC Use Table (Sec. 64-2-24.1); R-A and R-1 permit 1 dwelling per lot. By right only inside the Downtown Development District in T-3, T-4, T-5.1 and SD-WH (Appendix A use table). In R-2 a second unit is allowed as a Two-Family dwelling. Elsewhere an unlisted use needs a Planning Director determination (Sec. 64-2-24) or a BZA variance. Because approval is discretionary, confirm your own parcel with the planning department before budgeting. See our full Mobile ADU rules page.
ADU construction in Mobile typically costs $90,000-$175,000. The cost per square foot averages $120-$195. Garage conversions are more affordable at $40,000-$90,000, while prefab ADUs cost $70,000-$150,000. Permit fees add 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. to your budget. See our detailed Mobile ADU cost guide.
Detached ADUs in Mobile can be up to No ADU size cap is codified. Attached ADUs are limited to No ADU-specific cap; an attached second unit in R-2 counts as a Two-Family dwelling and must meet R-2 lot area. Junior ADUs (JADUs) have a cap of No JADU category in Mobile's UDC. Maximum height is District height limit: 35 ft in R-A, R-1 and R-2 (Secs. 64-2-4 to 64-2-6). Downtown, outbuildings are limited to 2 stories in T-3 and T-4 and to the principal building's height elsewhere (Appendix A). You are allowed up to R-A and R-1: 1 dwelling per lot on a qualifying lot.
Rear setback: 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. Side setback: District side yard: 5 ft in R-1 and R-2 (Urban and Suburban), 10 ft in R-A (Secs. 64-2-4 to 64-2-6). Front setback: District front yard: 5 ft in R-1 Urban and R-2 Urban, 25 ft in R-1 Suburban, R-2 Suburban and R-A (Secs. 64-2-4 to 64-2-6). Note that garage conversions may be exempt from setback requirements if the existing structure is already within the setback area.
1 space per Accessory Dwelling Unit (Table 64-3-12.1), on top of 2 per single-family dwelling; no minimum parking inside the Downtown Development District or Spring Hill Overlay (Sec. 64-3-12). Many cities and states have been relaxing ADU parking requirements in recent years. In California, no additional parking is required for ADUs near transit.
No owner-occupancy rule exists in the UDC for ADUs or two-family dwellings; any condition would come from a BZA variance or Director determination. Note that owner-occupancy rules vary by state law — California and several other states have eliminated this requirement for standard ADUs.
The ADU permit review timeline in Mobile is typically Not published. Build Mobile posts no target review time for residential plans; the residential checklist says to contact Permitting (251-208-7198) once all reviews are complete to schedule permit issuance. Plan check: Residential plan review through the CSS portal: no fee on first submittal, then $51, $101, $201 and $501 for the 1st-4th resubmittals; a $51 plan review fee applies in flood zones or for plans over 4,000 sq ft (Residential Plan Review Checklist, Jan. 2023). Pre-approved plans availability: None.
Average ADU rental income in Mobile: 1-bedroom units rent for $750-$1,100/month, 2-bedroom units for $900-$1,300/month. Short-term rentals: Short-Term Rentals (stays under 180 consecutive days) are a Conditional Use in R-A, R-1, R-2, R-3 and R-B, so a City Council Conditional Use Permit is required before listing any unit (Use Table Sec. 64-2-24.1; Art. Expected ROI: 5-8% annual return.
The minimum lot size for an ADU in Mobile is District minimums: R-1 Urban 6,000 sq ft, R-1 Suburban 7,200 sq ft, R-2 Urban 4,000 sq ft per dwelling unit, R-2 Suburban 8,000 sq ft, R-A 1 acre (Secs. 64-2-4 to 64-2-6). This applies to detached ADUs — garage conversions and JADUs may have different or no lot size requirements.
Garage conversions are one of the most popular and affordable ADU options. In Mobile, a garage conversion typically costs $40,000-$90,000. The key advantages are lower cost (no foundation work), potential setback exemptions, and faster permitting. Read our complete garage conversion guide.
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