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James Hardie Home Additions & ADU: The Miami Owner’s Guide
James Hardie home additions and ADUs in Miami typically run $53,100 to $153,400 installed, depending on square footage, foundation work, and the specific Hardie siding and trim lines you specify. We stock HardiePlank lap siding, HardiePanel vertical siding, HardieTrim boards, and HardieSoffit panels for additions of any size, and we can match existing Hardie products on your main house so the new structure doesn’t look like an afterthought. Most Miami homeowners call us after they’ve spotted moisture damage at the transition point where a previous addition meets the original exterior, or after a building inspector flags improper flashing on an older add-on.
The part that usually fails first isn’t the Hardie product itself. It’s the integration: the flashing, the weep screed, the gap between the bottom edge of the siding and the grade. In Miami, where afternoon thunderstorms dump an inch of rain in twenty minutes and humidity sits above 70% ten months of the year, a poorly integrated addition starts showing its age within two or three seasons. That’s what we fix, and that’s what we build to prevent.
Which James Hardie Models This Applies To
If your home or existing addition carries HardiePlank lap siding, HardiePanel vertical siding, HardieShingle, or HardieTrim boards, we can match product, profile, and color for the new structure. The trick is knowing exactly what’s on your walls now. HardiePlank comes in 5/4-inch and 4-inch exposures, and HardiePanel comes in 4×8, 4×9, and 4×10 sheets with smooth or stucco texture. Color matters too. Hardie’s ColorPlus finish is factory-applied and carries its own finish warranty, but if your existing siding was primed on site and painted, we’ll match with an approved exterior acrylic paint instead. The gap matters more than the board. In Miami’s wet season, that 1/4-inch clearance at butt joints and the 2-inch clearance above grade are what keep water out of the wall cavity. Most failed additions we see in Coconut Grove and Coral Gables skipped those clearances entirely.
We’re an independent James Hardie installer in Miami, not a factory branch. That means we’ll give you a straight recommendation between OEM Hardie trim and primed alternatives, and we’ll tell you when the Hardie product genuinely earns its cost and when a different material makes more sense for a non-visible area. Penhallow Remodeling is part of the Kingswell family of companies, and every Hardie addition we build gets the same written price guarantee and 365-day promise as any other project we touch.
- HardiePlank lap siding: 5/4″ or 4″ exposure, smooth or cedar texture, ColorPlus or primed
- HardiePanel vertical siding: 4×8, 4×9, 4×10 sheets, smooth or stucco texture
- HardieTrim boards: 4/4 and 5/4 thickness, used at corners, windows, and door surrounds
- HardieSoffit panels: Vented or non-vented, matched to the main house for visual continuity
- HardieShingle: Straight-edge or staggered-edge panels for accent walls and dormers
Styles & brands we work with
Signs Your James Hardie Home Addition or ADU Is Failing
- Wavy or buckling siding at the bottom edge. This is almost always a clearance problem – siding touching grade wicks moisture into the board. Hardie is cement-based and resists rot, but constant moisture will soften it and break the paint bond. In Miami’s clay soils, this shows up fast.
- Caulk cracking and peeling at trim joints. The builders either used interior caulk on the exterior or skipped backer rod entirely. Once water gets behind the trim, the wall sheathing starts absorbing it, and the problem spreads behind the Hardie.
- Rust streaks or staining below windows and at corners. Usually failed flashing or missing head flashing above window and door openings. The water running down the face of the wall hits the opening and gets routed inward instead of outward.
- Soft spots when you press a screwdriver against the sheathing, visible from the crawlspace or attic side. This is the late-stage symptom. By the time you can see interior staining, the sheathing needs replacement, not just the siding.
- Interior paint bubbling or peeling on walls that face the prevailing wind. Miami’s east-facing walls catch the most driving rain. If the weather barrier was installed poorly, you’ll see it inside before you see it outside.
See the difference
Before
After
Before
After
Before
AfterReplacement Cost & What’s Included
For a full James Hardie home addition, plan on $53,100 to $153,400 in Miami, depending on size, foundation type, and the level of finish. That range assumes proper permitting, structural engineering where required, Hardie siding and trim on the exterior, and interior finish to match the rest of the house. We write the price down before work starts. That’s not a courtesy – it’s how Trevor Nash has run every job since he learned the trade at Los Angeles Trade-Technical College and started swinging a hammer, and it’s how Penhallow Remodeling Miami operates now. The number on the written estimate is the number on the invoice.
What drives the cost up or down: square footage first, then foundation work. A slab-on-grade addition costs less than one requiring pilings or elevated construction. Bathroom and kitchen additions cost more per square foot than a bedroom or living space. ADUs fall at the upper end because they need their own mechanical systems and often their own panel. Every job gets a written line-item quote, a written 365-day promise, and all required Miami-Dade permits pulled and inspections passed before we hand you the keys.
Key Takeaways
- Installed home addition or ADU pricing: $53,100 – $153,400 in Miami
- Written price before work starts, written guarantee after
- Permits pulled, inspections passed – no exceptions
- 365-Day Done Right Promise: if it fails within a year, we come back and fix it at no charge
- Background-checked, uniformed crew on every job
DIY or Call Us?
Washing your Hardie siding twice a year and touching up paint is reasonable homeowner work. Rebuilding a wall, replacing sheathing, or recutting flashing on an addition is not. Hardie board is concrete – it’s heavy, it requires special blades and fasteners, and a wrong cut throws off the entire weather barrier. This isn’t a job where the danger shows up as a broken bone; it shows up six months later as mold inside the wall. If you’re fixing failed siding on an addition yourself, you’re likely covering up the underlying cause anyway, and the board you install today will be swelling before next hurricane season.
Trevor’s rule is simple: give you a number first, then pick up the tools. Call (850) 988-7559 and we’ll come out, look at what you have, match the existing Hardie product, and hand you a written price. Free estimate, no pressure, no surprise at the door.
FAQs
Properly installed and maintained, James Hardie siding on a Miami addition should last 30 to 50 years, but the paint or ColorPlus finish will need attention every 10 to 15 years in our sun and humidity. The structure behind it matters more – if the flashing and weather barrier were done correctly, the whole assembly lasts. If they weren’t, you’ll see problems in three to five years regardless of how good the Hardie product is. Call (850) 988-7559 for a free inspection and we’ll tell you which situation you’re in.
Yes, in many cases we can strip the bad siding, replace damaged sheathing, correct the flashing, and install new Hardie product only where it’s needed. If the failure is localized – say, one wall or a window opening – a repair is often the smarter spend. But if the integration problem runs the full perimeter, patching it piecemeal will keep failing in new spots. We’ll show you what we found and give you a written price for either path.
A small addition can be substantially complete in 10 to 14 weeks. A full ADU with plumbing, electrical, and kitchen or bath usually runs 16 to 24 weeks in Miami, with permitting time depending on the municipality. We’ll give you a project schedule in writing before work starts – not a vague timeline, but a real sequence with specific milestones – and we’ll tell you if anything slips and why.
We don’t warehouse Hardie inventory, but we order direct from local suppliers who do, and we spec the exact product, profile, and color needed during the estimate so there’s no substitution at rough-in. For fast-turn repairs, we can usually have HardiePlank and HardieTrim on site within a few days of the signed estimate. The brands we trust alongside James Hardie include KraftMaid, Cambria, and Silestone when your addition includes kitchen or bath work.
Call (850) 988-7559 to schedule a free estimate for your James Hardie home addition or ADU in Miami. A real person answers, and we’ll set a time that works for you.
Written by Trevor Nash, Owner, Penhallow Remodeling Miami at Penhallow Remodeling Miami, serving Miami and nearby areas.
