As of October 2026, there is no published cost study for Jacksonville ADUs alone. National HomeGuide figures run about $150 to $250 per sq ft for a detached ADU and $50 to $200 per sq ft for a garage conversion, and Angi's national detached range is $110,000 to $285,000 before permits. It depends heavily on flood elevation, utilities and finishes.
What do national cost guides report?
- Detached ADU: $110,000 to $285,000 (Angi); $150 to $250 per sq ft (HomeGuide).
- Garage conversion: $60,000 to $150,000 (Angi); $50 to $200 per sq ft (HomeGuide).
- Attached ADU: $100,000 to $216,000 (Angi); $125 to $225 per sq ft (HomeGuide).
- Above-garage unit: $128,000 to $225,000 (Angi); $200 to $500 or more per sq ft (HomeGuide).
Angi's ranges exclude permits and fees. Both sources are national, so they blend high- and low-cost regions. They are a starting point, not a Jacksonville price.
What does the size cap mean for cost?
The City of Jacksonville's 2022 ordinance was reported to cap an ADU's footprint at 25% of the main home's floor area or 750 sq ft, whichever is less. Small units cost more per square foot, because a kitchen and a bathroom cost about the same whether the unit is 400 or 750 sq ft.
What do Jacksonville lots add to the price?
- Flood elevation. On a lot in a special flood hazard area, raising the floor with stem walls, fill or piers.
- Wind design. Engineered connections, and impact-rated or protected openings in wind-borne debris regions near the coast.
- Septic or sewer. Septic capacity checks, or a long sewer run to the street.
- Electrical service. Many older homes need a panel or service upgrade to carry a second unit.
- Trees. Working around or permitting the removal of protected trees.
- Termites and moisture. Especially on garage conversions.
How should I compare quotes?
Ask each builder for a written, itemized quote that shows survey, design, engineering and permit fees; the foundation type and floor elevation; utility connections and who pays the utility; window and door ratings; finish allowances; and how changes are priced.
When does cost depend most on the lot?
It depends on two things a buyer cannot see: the elevation and what is underground. A lot that needs its floor raised two feet, or a septic system that cannot take another bathroom, can add more than any finish choice. We check the flood map and utilities before quoting.
Related
Sources, as of October 2026: Angi, "How Much Do ADUs Cost?"; HomeGuide, "How much does it cost to build an ADU?"; Jacksonville Daily Record on Ordinance 2022-0448. Figures are published estimates, not our quotes.
Updated October 2026