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James Hardie Home Additions & ADU: The San Antonio Owner’s Guide
If you’re planning a home addition or ADU in San Antonio and want James Hardie siding and trim on the exterior, a full project typically runs $45,000 to $130,000 installed, with ADUs landing between $90,000 and $220,000 depending on size, foundation work, and interior finish level. We’re an independent San Antonio remodeling contractor, not a James Hardie dealer, which means we buy Hardie products at trade pricing and install them to spec without pushing you toward one siding brand over another. Most homeowners find us when their original addition was built with cheaper fiber cement or vinyl, and after five to eight Texas summers, the corners are peeling, the trim boards are swelling, or water is getting behind the siding at the roofline.
What you need to know right away: James Hardie plank and panel products carry a 30-year limited warranty from the manufacturer, but that warranty only covers the material, not the labor to tear off and reinstall. The real cost question in San Antonio is almost never the Hardie board itself. It’s the framing underneath, the flashing details, and whether the previous crew built the addition to handle our clay soil and summer heat. Isaac Pruett has spent two decades fixing additions across the Southwest, and he’ll tell you flat out: the siding fails first at the places where someone cut a corner on flashing, not because the Hardie product gave out.
Which James Hardie Models This Applies To
James Hardie makes several product lines, and the right one for a San Antonio addition depends on what the rest of your house is wearing. The most common is HardiePlank lap siding, which comes in 4×8 and 4×10 sheets or individual planks in 5.25-inch, 6.25-inch, 7.25-inch, and 8.25-inch exposures. If your original addition used HardiePanel vertical siding, you’re looking at 4×8, 4×9, or 4×10 sheets, and matching the texture matters: Smooth, Cedarmill, or the newer stucco-look finish. HardieTrim boards come in 4/4 and 5/4 thicknesses, and here’s a gotcha most San Antonio homeowners don’t know: the 5/4 trim is actually 1 inch thick, not 1.25 inches like real lumber, so when you’re matching existing trim, you have to measure the actual board, not rely on the nominal size written on your original invoice.
For ADUs, Hardie’s Artisan line gives you a thicker plank with a deeper shadow line, but it requires furring strips or a rainscreen behind it in our climate. We stock HardiePlank, HardiePanel, and HardieTrim in the most common San Antonio colors on the truck, and we can order any of the 700+ ColorPlus finishes within three to five business days. Aftermarket equivalents from other fiber cement brands exist, but they don’t carry the same 30-year substrate warranty, and color matching between brands is never exact after two years of Texas sun.
Styles & brands we work with
Signs Your James Hardie Home Additions & ADU Is Failing
- Gaps at butt joints or corner trim pulling away. Hardie plank expands and contracts with humidity. If the original installer butted planks tight, they’ll cup in summer. The fix is re-gapping and re-caulking, but if the joints keep opening, the framing behind them is moving, and no caulk will hold.
- Bubbling or peeling paint at the bottom-most course. In San Antonio, this is almost always ground splash. Hardie requires a 6-inch gap between the bottom of the siding and grade. If soil or landscaping crept up over the years, water wicks up and the paint lifts first on the north and east walls where things stay damp longer.
- Cracked planks at outside corners. Hardie is cement board. It’s brittle. If a trim nail was driven too close to the edge or someone over-drove the nails with a framing gun set too high, the board cracks. This is a material handling issue, not a product defect, and it’s common in cheaper additions.
- Staining or dark streaks below window sills and at roofline intersections. That’s water getting past the flashing. Hardie sheds water, but it’s not waterproof by itself. If the original crew skipped metal head flashing over windows or the step flashing at the roof-to-wall junction, water runs behind the siding and stains it from the inside out.
- Soft spots in HardieTrim around door frames. Trim boards sit closest to the ground and get hit hardest by sprinklers and rain splash. If the end grain wasn’t sealed with primer before installation, the board swells and disintegrates. We see this on almost every addition built before 2015 in neighborhoods like Stone Oak and Alamo Heights.
See the difference
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AfterReplacement Cost & What’s Included
A full siding and trim replacement on a typical 300 to 600 square foot addition in San Antonio runs $14,000 to $32,000 depending on product line, trim complexity, and whether we’re tearing off the old siding or installing over it. A complete home addition build, including framing, Hardie exterior, roofing, drywall, electrical, and a basic interior finish, starts around $45,000 and tops out near $130,000 when you add a bathroom, kitchenette, or higher-end trim. Here’s how the numbers break down:
- HardiePlank siding replacement on an existing addition: $14,000 – $32,000 installed
- HardiePanel or vertical siding replacement: $12,000 – $26,000 installed
- Full home addition (400 – 800 sq ft), Hardie exterior included: $45,000 – $130,000
- ADU construction (600 – 1,200 sq ft), Hardie exterior included: $90,000 – $220,000
- Garage conversion to living space, Hardie siding on new walls: $30,000 – $80,000
What drives the range: the condition of the existing framing, whether we find rotted sheathing behind the old siding (common at the bottom plates), how much trim and corner work your addition has, and whether you’re matching a ColorPlus finish or painting on site. Every project gets a written price before we touch anything. No change orders unless you change the scope. The 365-Day Done Right Promise covers the work for a full year from completion. If something fails in that window, we come back and fix it at no cost, no dispute process.
DIY or Call Us?
What you can safely do yourself: check your gaps. Walk the addition and look at every place two Hardie planks meet, every corner, every place siding touches a window frame or door frame. If you see gaps wider than 1/8 inch, note them. Check your ground clearance. If dirt or mulch is within 6 inches of the bottom course, pull it back. That kind of inspection takes twenty minutes and costs nothing.
What you shouldn’t do yourself: pull off siding, replacement trim, or flashing. Hardie board installed incorrectly voids the manufacturer warranty, and the board is heavy enough that a dropped plank will break a toe or crack a slab. Working at the roofline on a ladder while handling 12-foot cement planks is how people get hurt. The safety rule here is simple: if it requires more than a caulk gun and a putty knife, call (254) 269-5071 and we’ll send a crew that’s background-checked, uniformed, and introduced to you by name and photo before they arrive.
FAQs
How long does James Hardie siding last on a San Antonio addition?
James Hardie siding lasts 30 to 50 years in San Antonio when installed with proper clearances, flashing, and paint maintenance, with the substrate warranty running 30 years from the manufacturer. The first place it fails in our climate is at ground contact and roofline intersections where flashing was skipped or done wrong, often within the first decade. If your addition’s siding is failing before year ten, it’s an installation issue, not a product problem. Call (254) 269-5071 and we’ll inspect it for free.
Can you repair a section of damaged Hardie siding instead of replacing the whole addition?
Yes, isolated damage from impact, sprinkler splash, or a single bad flashing detail can usually be repaired by replacing only the affected planks and trim boards, typically $800 to $3,500 for a San Antonio addition depending on how much we have to remove and whether we need to match an obsolete color. ColorPlus finishes fade in the Texas sun, so a patch may be visible for the first year until it catches up, but we’ll tell you that before we start. Call for a written quote on a section repair.
How long does it take to replace Hardie siding on an addition?
A full siding and trim replacement on a typical San Antonio home addition takes 5 to 10 working days depending on weather, the amount of trim detail, and what we find behind the old siding. If we uncover rotted sheathing or framing, we’ll show you photos, tell you what it costs, and get written approval before we proceed. You’ll get a schedule before we start and a final walkthrough when we’re done.
Do you stock James Hardie siding and trim, or do we have to order it?
We stock HardiePlank in the most common San Antonio sizes and textures, HardieTrim in 4/4 and 5/4, and the fasteners and flashing we use on every job in our trucks and warehouse. ColorPlus pre-finished products in less common colors take 3 to 5 business days to order. We also work with KraftMaid, Cambria, and Silestone for the interior side of addition and ADU projects if you’re building from scratch.
If you’re standing in front of a San Antonio addition with failing siding, cracked trim, or water stains that won’t dry out, call (254) 269-5071 for a free estimate. A live person answers. We’ll send a written price before any work starts, and Isaac Pruett will make sure you know exactly who’s coming, what it costs, and what’s covered before we ever put a ladder against your wall.
Written by Isaac Pruett, Owner, Mason Remodeling Co San Antonio at Mason Remodeling Co San Antonio, serving San Antonio and nearby areas.
