James Hardie Junior ADU Conversion: The San Antonio Owner’s Guide
James Hardie junior ADU conversion in San Antonio typically runs $14,500 to $28,000 installed, depending on whether we’re working with HardiePlank lap siding, HardiePanel vertical siding, or HardieTrim boards around a converted garage or backyard structure. We stock Hardie-compatible trim, panel, and flashing components on our trucks for most San Antonio jobs, and we can usually start within a week of your written estimate. Most owners discover a problem the same way: a corner of siding pulls loose after a spring storm, water stains appear under a window, or the bottom edge of a panel starts to swell and crumble where it meets the slab.
You’ll hear this from me directly, because I’ve seen it too many times on conversions across San Antonio: a failed Hardie part on a junior ADU usually means water got where it wasn’t supposed to, and now the framing behind it is asking questions. The fix is rarely just the visible board. It’s the flashing, the house wrap, and the sealant behind it. If you call me about a crumbling corner, I’m going to ask what the wall looks like inside before I quote you a board swap. That’s not upselling. That’s doing the job once.
Key Takeaways:
- Installed cost in San Antonio typically ranges from $14,500 to $28,000 for a complete conversion.
- We are an independent builder, not affiliated with James Hardie, but we install their products daily.
- Most failures start at the bottom edge or around windows where water sits against the siding.
- You get a written price before any work starts, and the 365-Day Done Right Promise covers the finished job.
Which James Hardie Models This Applies To
We work with the full James Hardie siding and trim line on junior ADU conversions: HardiePlank lap siding, HardiePanel vertical siding, HardieTrim boards, and HardieSoffit panels. The right part depends on what was originally installed, and here’s the compatibility gotcha most people miss: HardiePlank comes in 5/16-inch and 7/16-inch thicknesses, and the 7/16-inch version only works with specific trim profiles. If you try to match a 5/16-inch lap board with trim meant for the thicker panel, you’ll get a lip that catches water and fails again within a few San Antonio rain seasons. We measure before we order anything.
To identify your parts, look for the print line on the back of the board or the edge of the trim. Hardie stamps its products with a code that includes the thickness and profile. If you can’t read it because the board is installed, we’ll pull a small section and check during the estimate. For most San Antonio conversions built in the last ten years, you’re looking at HardiePlank lap siding with ColorPlus factory finish. The OEM route is fine, but we routinely use quality aftermarket equivalents from Versetta Stone and Nichiha when a panel is discontinued or the original color is no longer on the line. You get the same 50-year substrate warranty, and we match the existing color closer than a factory respray.
One more San Antonio reality: Hardie says 50 years on the material, but the sealant joints and flashing on a conversion here rarely make it past 8 to 12 years. The Texas sun bakes caulk until it cracks, then the next heavy rain does the rest. If your junior ADU is past the ten-year mark and you’re seeing frequent touch-up needs, the smart move is a full re-flash and repaint while we have the walls open.
Signs Your James Hardie Junior ADU Conversion Is Failing
- Swelling or crumbling along the bottom edge. Water wicks up from the ground where siding sits too close to the slab or grade. In San Antonio’s clay-heavy soil, shifting ground can push panels tight against the foundation, trapping moisture. Once the bottom inch goes soft, the board is done.
- Paint peeling in a straight vertical line under a window. That’s not a paint problem, it’s a flashing problem. The head flashing above the window is likely missing or installed wrong, and water is running down the wall behind the siding. The Hardie board is fine; what’s behind it isn’t.
- Gaps opening at corner trim joints. Hardie boards expand and contract with San Antonio’s temperature swings. If the original installer didn’t leave a proper joint gap and backer rod, the corners start pulling apart. You’ll see daylight through the gap before you see water inside.
- Bubbling or soft spots under the paint surface. This usually means the substrate behind the siding has started to rot and the moisture is working outward. By the time you feel it with your hand, the sheathing under the Hardie is compromised.
- Visible mold streaks on the north-facing wall. San Antonio homes get shade on the north side almost year-round, and hardie holds moisture there longer than a south wall. If you’re cleaning mold off the same wall every spring, the siding is doing its job but the wall assembly isn’t breathing properly.
Replacement Cost & What’s Included
A junior ADU conversion with James Hardie products in San Antonio typically lands between $14,500 and $28,000 installed. Here’s what drives the number:
- Base HardiePlank conversion with trim and flashing: $14,500 to $18,000 for a standard 400 to 600 square foot structure.
- HardiePanel vertical siding with rainscreen install: $19,000 to $24,000 depending on wall height and window count.
- Full tear-off, re-sheathing, new Hardie siding, trim, and paint: $24,000 to $28,000 for a conversion with hidden rot or failed sheathing.
Every quote includes the line-item cost for materials, labor, flashing, house wrap, sealant, and disposal. You’ll see the price before we touch a wall, and that price doesn’t change once work starts unless we uncover something structural and you approve the change in writing. That’s the deal we make with every family in San Antonio. It’s also why the 365-Day Done Right Promise is on paper for every job: if something isn’t right a year later, we come back and make it right. No fine print, no argument.
DIY or Call Us?
You can safely check the caulk joints, look for gaps at trim edges, and photograph anything unusual before calling. You should not attempt to remove Hardie siding yourself. The boards are heavy, they’re held with nails into framing you can’t see, and prying one loose often cracks the board above it. More importantly, cutting Hardie releases silica dust that requires proper respiratory protection, and installing it wrong will void the material warranty and invite the same leak you’re trying to fix. This is not a weekend project. If the bottom edge is crumbling or a corner has pulled loose, call us at (210) 981-6358. Tell me what you need the space to do, and we’ll figure out the rest from there.
FAQs
The Hardie material itself carries a 50-year substrate warranty, but the sealant, flashing, and paint on a San Antonio conversion realistically last 8 to 12 years before they need reapplication. Our installs include a written warranty on the workmanship through the 365-Day Done Right Promise, and we’ll walk you through the maintenance schedule at handoff. Call (210) 981-6358 for a specific timeline on your structure.
Yes, if the damage is limited to one or two boards and the sheathing behind them is dry and solid. We cut out the damaged section, inspect what’s behind it, install new flashing and matching Hardie boards, and re-seal the joints. If the rot has spread to the framing or the sheathing is compromised, a panel-by-panel patch will fail again. We’ll show you photos either way before work begins.
A standard junior ADU siding conversion in San Antonio takes 5 to 10 working days, depending on whether we’re doing a spot replacement or a full tear-off with re-sheathing. We’ll give you a day-one schedule and stick to it, and the crew will leave the site clean at the end of every workday. Shoe covers and drop cloths inside the structure are standard, and you’ll get a photo of the clean-up before we head home.
We stock common HardiePlank, HardiePanel, HardieTrim, and flashing profiles on our trucks for San Antonio jobs, including the 5/16-inch and 7/16-inch lap boards and standard trim pieces. If your conversion needs a discontinued color or a specialty panel, we’ll order it and have it within a few days. We also work with Abodu, Cover, and Dvele products when a conversion calls for a prefab approach. Call (210) 981-6358 and we’ll tell you what’s on the truck today.
Written by Ben Ortega, Owner, Seedling ADU San Antonio at Seedling ADU San Antonio, serving San Antonio and nearby areas.
