Home Additions & ADU in Key Largo - One Fixed Price, Permits Handled

Fast, Reliable Home Additions & ADU Across Key Largo

Home addition and ADU construction in Key Largo runs roughly $45,000 to $130,000 for a typical room addition, while a detached ADU lands between $90,000 and $220,000. Those numbers reflect something every Key Largo homeowner already suspects: building on coral rock with Monroe County’s flood and hurricane codes is a different animal than the Florida mainland. At Barrett & Sons Construction, we’ve been owner-operated since 1998, and our crews work Key Largo regularly enough to know the difference. If you’re planning a second story, a garage conversion, or a new detached ADU, call Tom and the team at (877) 610-1318 for a free estimate. We’ll give you real numbers in writing before any work begins.

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Key Takeaways

• Typical room addition in Key Largo: $45,000-$130,000
• Detached ADU: $90,000-$220,000; garage conversion: $30,000-$80,000
• Every Key Largo addition requires driven pilings anchored into coral rock, not slab footings
• Monroe County ROGO permit rationing and HVHZ wind codes add steps mainland contractors skip
• Barrett & Sons pulls every permit, passes every inspection, and backs work with a 90-Day Done Right Promise

Why Barrett & Sons Construction Is Key Largo’s Preferred Home Additions & ADU Company

Tom Barrett founded this company in 1998 and put his name on the truck. It’s still his name on the truck today, and his sons run the crews on most days. That matters in Key Largo because the guy who shakes your hand on Monday is the same guy on the job site Wednesday. We don’t dispatch strangers from an out-of-state call center, and when you call, a live person answers 24/7.

We’ve done enough work in Key Largo to know that the permitting process here is unlike anywhere else we build. Monroe County’s ROGO system ration permits, and the county floodplain review runs parallel to the HVHZ wind code review. A contractor who’s never pulled a permit in Key Largo will stall on week two and hand you change orders for work that should’ve been in the original scope. We put scope, allowances, and every change order in writing before a single nail moves. Ask anyone in the 33037 ZIP code who’s built here, or look up Tom’s license number yourself. His name and address are published.

Our Home Additions & ADU Services in Key Largo

Here’s what we handle most often on the island, from Home Additions & ADU design through final inspection:

ADU Design & Permitting

This is the service that catches out-of-county contractors off guard. A legal ADU in Key Largo starts with an engineered plan that accounts for coral rock, base flood elevation, and Monroe County’s HVHZ load paths. We handle the drawings, the county floodplain review, and the ROGO allocation paperwork so your permit doesn’t stall. We can’t shortcut the timeline, but we can make sure you’re not the one calling the county every Friday.

New Detached ADU

A detached ADU in Key Largo is a full build: driven pilings, stainless steel hardware, impact-rated openings, and a roof assembly engineered for 150+ mph wind. We recently completed one off Tarpon Basin Drive in the 33037 ZIP for a homeowner whose 1980s CBS stilt house needed a separate in-law suite. The job meant driving six pilings through coral rock for the ADU foundation, then framing the entire structure to Monroe County’s HVHZ wind code. We used all stainless-steel anchor bolts and straps, and our crew passed the county floodplain review and inspection sign-off before we ever started rough-in. That’s the standard, not the exception.

Garage Conversion ADU

Converting a ground-level garage in Key Largo usually means contending with flood elevation requirements. If the existing slab sits below base flood elevation, the county will not sign off on a living space without remediation. We’ve handled enough garage conversions in AE and VE flood zones to know what triggers a redesign and what passes first review. Expect $30,000 to $80,000 depending on elevation work, electrical, and whether you’re adding a bathroom or kitchenette.

Junior ADU & Interior Conversion

For homeowners who want an in-law suite without a separate structure, an interior conversion can work if the main house already sits high enough above base flood elevation. These projects tend to move faster through the county because they don’t require new pilings, but they still need the floodplain and HVHZ review. We’ll walk the property first and tell you honestly if a junior ADU is viable or if you’re better off with a detached build.

Trusted Brands We Service in Key Largo

For impact windows and doors on Key Largo additions and ADUs, we install PGT and CGI products. These are the names Monroe County inspectors know, and they hold up under HVHZ testing. We also spec Simonton where the window package calls for it. Salt air is brutal on cheap hardware, so we don’t install mystery brands off a van. When you sign a contract with Barrett & Sons, the window and door brands go in writing alongside the prices.

Common Home Additions & ADU Problems We See in Key Largo Homes

• Slab-on-grade quotes from mainland contractors. The island sits on Key Largo Limestone, fossilized coral reef just a few feet below grade. Conventional trench footings are physically impossible here. Any contractor who quotes slab-on-grade for an addition has never built in Key Largo. You’ll find out when the rock auger hits coral at two feet and the permit gets revised for driven pilings.
• Second-story additions on pre-1992 stilt homes without structural retrofits. Homes built before Hurricane Andrew often lack code-compliant roof-to-wall connections and continuous load paths. Adding weight on top of those old straps can overload them. The dual flood and HVHZ review will force a full structural retrofit before sign-off. We flag this in the estimate, not after demo starts.
• Enclosed patios and garage conversions below base flood elevation. If the finished floor sits below the flood elevation line, the county floodplain review fails. Remediation can mean raising the floor, adding fill, or redesigning the entire conversion. That’s a change order that should’ve been a line item from day one.
• Corrosion from salt air on standard hardware. Key Largo sits between the Atlantic and Florida Bay with no inland buffer. Standard galvanized fasteners corrode fast here. We spec stainless steel or hot-dipped galvanized anchor bolts and straps as the baseline, not an upgrade. It’s written into every Key Largo scope we price.

Pricing for Home Additions & ADU in Key Largo, FL

We Also Serve Cities Near Key Largo

Barrett & Sons Construction serves homeowners throughout Key Largo and the surrounding South Florida communities, including Florida City, Homestead, Leisure City, and Naranja. Crews are coordinated from our Miami-area office, so scheduling a walkthrough or estimate in Key Largo is straightforward. If you’re in the 33037 ZIP or anywhere nearby, call us and we’ll get you on the schedule.

FAQs - Home Additions & ADU in Key Largo