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James Hardie Home Additions & ADU: The Miami Owner’s Guide
A James Hardie home addition or ADU in Miami typically runs $53,100 to $153,400 installed, depending on square footage, foundation work, and whether you’re building up or out. We stock the full James Hardie siding, trim, and soffit line on our trucks and can usually start within two weeks of permit approval. Most Miami owners call us after noticing something smaller first: cracked HardiePlank lap siding near a leaking window, swollen HardieTrim boards at the bottom of a wall, or gaps where an old addition meets the original structure. That’s how it starts. Then the water gets in, and a $2,000 siding repair becomes a $60,000 wall rebuild.
Which James Hardie Models This Applies To
Our home addition and ADU work covers every current James Hardie exterior product: HardiePlank lap siding (5/16 in. and 3/8 in. thicknesses), HardiePanel vertical siding, HardieShingle, HardieTrim boards (4/4 and 5/4), HardieSoffit, and the ColorPlus factory-finished line. The key identifier is the product code printed on the back of each board: HardiePlank reads “HZ10” for the 5/16 in. profile, “HZ5” for the 3/8 in. The “HZ” means HardieZone, and it matters more in Miami than almost anywhere else in the country. James Hardie makes two regional formulas. The HZ5 and HZ10 products are engineered for the humid, wind-driven-rain, hurricane-cycle conditions we live with here. If you buy boards meant for the western climate zone online and a Miami contractor installs them, the warranty won’t cover the failure. We use only Florida-zone Hardie products on every addition and ADU we build. For ColorPlus boards, the finish carries a 15-year limited warranty when installed correctly. For primed boards, you’re painting on-site, and that’s a decision we’ll walk you through because Miami’s UV and salt air eat cheap paint in about four years.
Styles & brands we work with
Signs Your James Hardie Home Additions & ADU Is Failing
Most of what we tear out in Miami wasn’t installed by someone who understood water management. The signs are consistent:
- Swollen or crumbling HardieTrim at the bottom edge. This is almost always a clearance issue. James Hardie requires 6 inches between siding and grade, and 2 inches between siding and any hard surface like a walkway or roof intersection. Miami’s heavy rains splash back hard, and boards cut too close wick water like a sponge.
- Cracked lap siding around window corners and door headers. This is a nailing problem, not a product failure. HardiePlank is brittle when it’s nailed too tight or too close to an edge. The fix is re-flashing the opening and replacing the cracked boards, never caulking over the crack.
- Gaps opening between the original house and the addition. Miami sits on limestone and sandy fill. Footings move. If your addition wasn’t tied to the existing structure with proper control joints, the gap is the first visible symptom of differential settlement.
- Mold staining on interior ceilings under the roofline of the addition. Hardie siding doesn’t rot, but the framing behind it does. This usually means the housewrap or flashing was lapped wrong, and water has been getting behind the siding since year one.
- Faded, chalky, or peeling ColorPlus finish on the south and west exposures. Miami’s sun is brutal. Factory finishes fail predictably when early installation scratched the coating or exposed a cut edge. The substrate is fine. The protection is gone.
See the difference
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AfterReplacement Cost & What’s Included
Every Miami home addition and ADU we price includes the full James Hardie exterior package: siding, trim, soffit, housewrap, flashing, and paint or factory finish. The range on a home addition runs $53,100 to $153,400. The driver is square footage and foundation type. A simple 400-square-foot single-story addition with slab-on-grade in Kendall usually lands near the bottom. A two-story addition with a full second-floor build over an existing house in Coconut Grove can push the top of that range fast. An ADU is a different scale entirely: $106,200 to $259,600. That’s a full dwelling unit with kitchen, bath, separate meter, and its own Hardie exterior. We do not give verbal estimates that grow on the final bill. You get a written line-item price before any work starts. If we miss something in our own quote, that’s on us, not you. Every project includes the 365-Day Done Right Promise: if the work isn’t right within a year of completion, we make it right at no cost. That covers siding, flashing, trim, paint, and any water intrusion traced to our installation.
DIY or Call Us?
You can safely inspect your own Hardie siding. Check the ground clearance with a tape measure. Look for cracked boards around windows. Look for missing or pulled caulk at trim joints. Touch the bottom edge of the HardieTrim; if it feels soft or spongy, water is getting in. What you should not do is pull off boards to “see what’s behind them.” Once you remove siding, you’ve opened your building envelope, and Miami’s afternoon downpour doesn’t wait for you to figure out how to re-flash it. Hardie products also require specific fasteners and placement: roofing nails or siding nails at 16 inches on center on studs, never into sheathing alone. We’ve torn out additions where someone used drywall screws to hang HardiePlank. Every board sagged. Every fastener rusted. Call us at (850) 780-8123 and tell us what you’re seeing. We’ll give you a straight read over the phone and a written number before we touch anything.
FAQs
James Hardie siding carries a 30-year non-prorated limited product warranty, and in our experience in Miami’s sun, rain, and salt air, properly installed HardiePlank with correct clearance and flashing lasts 30 to 40 years before you’re painting or replacing sections. The first thing to fail is usually the paint or caulk, not the board itself. Call (850) 780-8123 for an honest assessment of what’s actually in good shape and what isn’t.
Yes, if the problem is caught early: cracked boards get replaced, flashing gets re-lapped, clearance gets corrected. What can’t be repaired is the framing behind the siding once rot and mold have set in. That’s why we tell Miami homeowners to call us the week they see a crack or a stain, not the year after. The repair is $2,000. The rebuild is $60,000.
A full exterior re-side on a 300-to-500-square-foot addition takes our crew four to seven working days, weather allowing. Miami storms will add a day here and there. We schedule around the forecast and don’t leave open walls overnight.
We stock HZ10 HardiePlank lap siding in 4×8, 4×9, and 4×10 configurations, HardieTrim in 4/4 and 5/4, HardieSoffit, and a supply of ColorPlus touch-up kits for the most common Miami exterior colors. We are an independent contractor, not affiliated with James Hardie, but we buy direct and order to spec for every job. If you’re matching an existing color, bring us a photo and we’ll match it before we ever schedule the install. Call (850) 780-8123 for a free written estimate.
Written by Isaac Pruett, Owner, Mason Remodeling Co Miami at Mason Remodeling Co Miami, serving Miami and nearby areas.
