Free written scope consultation - allowances, timeline, and change-order rules on paper before a single trade is scheduled
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AfterFast, Reliable Home Additions & ADU Across Key Biscayne
Adding square footage on Key Biscayne is not like adding square footage anywhere else in Florida. The island sits behind one bridge, inside the strictest building-code zone in the country, and on ground that is mostly under six feet of elevation. A home addition here runs $45,000 to $130,000 on a typical project, and an ADU runs $90,000 to $220,000 depending on whether you are converting a garage or building a detached unit from the slab up. If you are planning space for aging parents, a returning adult child, or rental income, call Barrett & Sons Construction at (877) 610-1318 and we will walk you through what your lot can actually support.
We have been doing this work since 1998, and we know what the Key Biscayne building department is going to ask for before you ever sign a contract. That matters here more than in most places, because a stalled permit on Key Biscayne means every material delivery has to re-cross the Rickenbacker Causeway on a new schedule. We build that reality into our timelines from day one.
Why Barrett & Sons Construction Is Key Biscayne’s Preferred Home Additions & ADU Company
Tom Barrett founded this company in 1998, and his sons run the crews. When we say you will know the people doing your work, we mean it. Tom publishes his name, his address, and his license number, and he wants you to look it up before you sign anything. That is how we have operated for over 29 years, and it is the reason Key Biscayne homeowners keep calling us back for second floors, garage conversions, and detached ADUs.
Our Key Biscayne customers tell us the same thing again and again: they were burned before by a company that quoted one number on the phone and showed up with a different one. We do not do that. Every allowance, every scope line, and every price change arrives as a written change order before a single nail moves. Our 90-Day Done Right Promise is written on every contract, and if something is not done right, one call brings Tom back to make it right.
We also answer our own phone. A live person picks up 24 hours a day, seven days a week, not a voicemail box and not an out-of-state call center. When you call (877) 610-1318 to ask about a home addition in Key Biscayne, you are talking to someone from our local South Florida team who knows the island.
Our Home Additions & ADU Services in Key Biscayne
ADU Design & Permitting
An ADU in Key Biscayne starts with a hard question: does your lot meet Miami-Dade’s setback, impervious-surface, and flood-zone requirements? Most island properties sit in FEMA AE or VE zones, which means the finished floor has to sit above base flood elevation. We handle the design, the elevation planning, and the full permitting package so the plan-review office sees a complete submittal on the first pass. Builders who mostly work in Broward or Palm Beach often miss the extra Miami-Dade NOA requirement. We do not, because we pull permits on every eligible job and the permit inspector’s name matters to us.
Garage Conversion ADU
A garage conversion is the most cost-effective path to an Home Additions & ADU on Key Biscayne, and our team typically handles these projects in the $30,000 to $80,000 range. Many island homes built between 1993 and 2008 have concrete-block garages that convert cleanly, but the flood rules still apply: mechanical systems have to move above base flood elevation, and the existing slab may need flood venting work. We have done enough of these on Key Biscayne to know exactly what the inspectors will flag.
New Detached ADU
A new detached unit on Key Biscayne, running $90,000 to $220,000, gives you a separate address for a tenant or a quiet place for family without sharing walls. Every concrete truck and framing delivery has to cross the Rickenbacker Causeway, so we sequence materials carefully and stage them on the lot rather than scheduling multiple trips. We also build to HVHZ standards throughout: impact-rated windows, proper tie-downs, and structural fasteners that carry Miami-Dade NOA approval, not just a generic Florida stamp.
Junior ADU & Interior Conversion
A junior ADU or an interior conversion inside your existing footprint is usually the fastest and least expensive way to add an independent living space in Key Biscayne. We convert ground-floor bonus rooms, split a large primary suite, or build out an enclosed patio as a comfortable in-law suite. Interior work moves faster through permitting because the footprint and the elevation do not change, but the same rules on egress windows, smoke separation, and independent entry still apply.
What happens when you call
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A real person answersNo phone trees - you reach a local pro.
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You get an upfront price rangeHonest numbers before anyone is dispatched.
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A background-checked tech heads outLicensed & insured, dispatched right away.
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You approve before work beginsNothing starts until you say go.
Trusted Brands We Service in Key Biscayne
When your Key Biscayne addition or ADU needs a hardscape connection, we bring the same premium-brand depth we use across every trade. We install Techo-Bloc and Cambridge Pavers on driveways, walkways, and patio surrounds, and we use Quikrete and Oldcastle products for concrete and masonry work that has to hold up to salt air and standing water. We do not put mystery materials off the back of a van on your property. Every product we specify is one you can look up before work starts.
Common Home Additions & ADU Problems We See in Key Biscayne Homes
- Flood-zone corrections on older slabs: Homes built in the 1990s and 2000s often have finished floor elevations that predate today’s stricter flood maps. A room addition can trigger a requirement to raise mechanical systems, add flood vents, or elevate the new slab, and we design for that from the first site visit so it does not become a surprise change order.
- Salt-corrosion damage in block construction: Decades of salt spray accelerate rebar corrosion in concrete, especially in older oceanfront properties. When we tie a new addition into an existing wall, we inspect the embedded steel and specify marine-grade materials where the exposure is worst.
- NOA confusion on windows and doors: A product stamped “Florida Product Approval” is not automatically legal in Key Biscayne. The permitting office requires a Miami-Dade County Notice of Acceptance for every window, door, and structural fastener, and our clients in Key Biscayne avoid rejection delays because we file with the right documentation from day one.
- Single-bridge logistics: Every dumpster, crane, and lumber flatbed on Key Biscayne crosses the Rickenbacker Causeway. We plan material delivery in full truckloads and coordinate crane windows in advance, because a missed delivery on the mainland is a two-hour delay here at best.
Pricing for Home Additions & ADU in Key Biscayne, FL
Here is what you are looking at in real Key Biscayne numbers, not vague ballparks.
- Room addition: $45,000 to $130,000
- Second-story addition: $60,000 to $130,000+ depending on structural load and interior finish
- Detached ADU (new build): $90,000 to $220,000
- Garage conversion ADU: $30,000 to $80,000
- Junior ADU / interior conversion: $25,000 to $60,000
- Sunroom or enclosed patio build-out: $25,000 to $65,000
Your price moves with foundation work, flood elevation corrections, structural reinforcement for second stories, and the level of interior finish. What does not change is how we quote: line-item, in writing, with every allowance spelled out. Call (877) 610-1318 to schedule a free estimate on your home addition in Key Biscayne, and we will put the numbers on paper before you commit to anything.
We Also Serve Cities Near Key Biscayne
If you are on the mainland just across the Rickenbacker Causeway, we bring the same standards to Coconut Grove, Coral Gables, Miami Beach, and Miami. Many of those homeowners call us after a mainland contractor stumbles on a permitting issue that our Home Additions & ADU team handles routinely. And when a Key Biscayne addition grows into a full-home project, we also handle Kitchen Remodeling in Key Biscayne and Bathroom Remodeling in Key Biscayne under the same contract and the same 90-Day Done Right Promise.
Serving Key Biscayne, FL - Our Local Coverage Area
We’re based in the Key Biscayne area and know this community well. Use the map below to see our service coverage - if you’re nearby, we can almost certainly help.
FAQs - Home Additions & ADU in Key Biscayne
Permitting is the pacing factor on Key Biscayne, not our crew schedule. Once your plans are approved and the permit is in hand, we can typically mobilize within two to three weeks. On a second-story addition or detached ADU, the structural review and flood-zone documentation add time up front. Call (877) 610-1318 and we will tell you honestly where your project is likely to slow down and where it will not.
Yes. We work across all of Key Biscayne, 33149, from the single-family streets near the Village Green to the high-rise condominium communities along the ocean side. Interior conversions inside condominium buildings carry their own association approvals and structural limitations, and we navigate those with the same written-change-order discipline we use on every single-family addition.
It usually helps with the structure and hurts with the paperwork. Post-1992 homes on Key Biscayne were built to strong concrete-block standards with impact-rated openings and hip roofs, which makes tying a new addition into the existing structure more straightforward. The challenge is flood elevation: many 1990s slabs no longer meet today’s FEMA AE or VE requirements, so the addition may trigger the need to elevate mechanical systems or add flood venting. We identify that in the first site walk so it is in your written estimate, not a surprise later.
Yes, typically by 8 to 15 percent. The single-bridge logistics add material delivery cost to every project on the island, and the Miami-Dade NOA requirement eliminates some cheaper product options that mainland builders can install without a second thought. You are also paying for the discipline of getting the permit right the first time on an island where a rejected submittal means another full crossing over the Rickenbacker Causeway. Our free estimate includes all of that, so call (877) 610-1318 for a real number.
Every Barrett & Sons job carries our 90-Day Done Right Promise, written on the contract. If something is not done right, Tom comes back and makes it right with one phone call. On structural work, manufacturer warranties from the window, door, and roofing brands we install run far longer, and we document every one of them for you at closeout.
We handle the full scope. One contract, one schedule, one point of accountability. That is especially important on Key Biscayne, where coordinating separate trades across the causeway creates its own delays. Our crews include the trades needed for your addition or ADU, and every permit and inspection is our responsibility. Call (877) 610-1318 to schedule your free estimate, and we will bring the full plan to your kitchen table.
Written by Tom Barrett, Owner at Barrett & Sons Construction, bringing our Home Additions & ADU team to Key Biscayne since our early years in South Florida.