James Hardie New Detached ADU: The Woodland Hills Owner’s Guide
If you’re pricing a new detached ADU clad in James Hardie siding in Woodland Hills, the honest installed range is $280 – $420 per linear foot for a turnkey build, which puts a 700 – 900 square foot unit between $195,000 and $380,000 depending on site work, foundation type, and utility runs. We stock the James Hardie plank and panel profiles on our own trucks, and we quote the job in writing before anyone swings a hammer. Most Woodland Hills homeowners call us because their existing siding failed at the butt joints, swelled at the bottom edge, or a previous contractor flashed the window heads wrong and water got behind the lap.
Key takeaways:
- A new detached ADU in Woodland Hills runs roughly $280 – $420 per square foot, all in, depending on site conditions and utility connections.
- James Hardie plank comes in specific exposures (4″, 5″, 6″, 7″) and you cannot mix profiles mid-wall without visible seams and premature joint failure.
- Hardie products fail fastest when installation breaks the manufacturer’s required 6″ ground clearance or when joints are caulked instead of flashed.
- We carry HardiePlank lap siding, HardiePanel vertical siding, and the matching trims on our trucks serving the West Valley.
Which James Hardie Models This Applies To
We install and stock parts for the full James Hardie siding line you’d spec on a detached ADU: HardiePlank lap siding (5/16″ thickness, exposures from 4″ to 7″), HardiePanel vertical siding (4’x8′, 4’x9′, and 4’x10′ sheets), HardieTrim boards (5/4″ and 4/4″ thickness), and HardieSoffit panels. The compatibility gotcha most owners miss: a 5/16″ HardiePlank lap needs the matching 5/16″ starter strip; using a generic starter strip leaves the bottom course kicked out at the wrong angle, which cracks the plank within two seasons of Woodland Hills sun loading.
For a new detached ADU, we typically recommend 7-1/4″ HardiePlank lap (6″ exposure) with ColorPlus technology. The factory finish holds up better in our UV exposure than field painting, and the 6″ exposure gives you a shadow line that reads as a traditional Los Angeles accessory dwelling, not a box dropped in a backyard. We are an independent contractor and not affiliated with James Hardie, but we’ve installed their siding on ADUs and room additions across Woodland Hills long enough to know which profiles hold paint and which ones telegraph every framing irregularity. Aftermarket equivalent trim (generic fiber cement from CertainTeed or Nichiha) can work, but the color match to a Hardie factory finish will drift within five years. For ADU projects, we spec OEM James Hardie trim when the siding is factory-finished ColorPlus.
Signs Your James Hardie New Detached ADU Is Failing
- Wicking at the bottom edge. Fiber cement will pull water like a candle wick if the bottom course sits closer than 6″ to grade or if the drip edge was left off. You’ll see a darkened band, then delamination, then crumbling. We see this on Woodland Hills lots where the slope pitches toward the ADU pad and the previous builder skipped the drainage swale.
- Cracking at the butt joints. James Hardie instructions are explicit: butt joints get a 1/8″ gap and flashing behind them, never caulk. When a joint is caulked, the plank can’t move with thermal expansion and it cracks at the fastener line. You’ll see vertical cracks running right through the face of the plank, usually at the joint closest to the corner.
- Nail heads popping through the face. Overdriven fasteners are the most common installation defect we rectify. A Hardie plank fastened with the nail head sunk below the surface will blow the face off within a year. The fix isn’t a patch; it’s replacement of that plank and fastening to the correct depth.
- Window head flashing failures. When the head flashing above a window is omitted or installed backwards, water leaks behind the siding and rots the sheathing. You won’t see the failure on the siding at first; you’ll see streaks of rust or mineral staining below the window trim, then the trim cups and pulls away.
- Mold or mildew lines at the corners. Fiber cement corners need a proper corner board or a mitered joint with flashing. A butt-fit corner left open will collect moisture and show a green-black stain line within two winters in Woodland Hills, especially on the north and east walls that never fully dry.
Replacement Cost & What’s Included
In Woodland Hills, a partial siding repair on a James Hardie-clad ADU runs $1,200 – $3,800 depending on how many planks need to come off and how much sheathing damage is behind them. A full re-side of a detached ADU runs $18,000 – $45,000 including tear-off, waste handling, new flashing, and paint or factory-finished replacement. The spread comes down to whether we’re replacing a single elevation, whether the sheathing is sound, and whether the existing siding was field painted or factory finished.
Every quote from Fielding ADU Woodland Hills includes the price in writing before we start. That price covers all materials, labor, flashing, joint treatment, and cleanup. It does not change unless we open the wall and find rot no one could have seen from the outside, and even then we document it with photos and get your sign-off on the change order before proceeding. The work is covered by The Garrison Standard: if a workmanship defect shows up inside one full year, we return and correct it at no charge.
DIY or Call Us?
You can safely check a few things yourself. Confirm the bottom course has at least 6″ of clearance to grade. Look at the butt joints and see whether they’re gapped and flashed or smeared with caulk. Check that your irrigation spray doesn’t hit the siding on a regular cycle, because Woodland Hills water is mineral-heavy and it will etch the finish over time.
What you should not do is pull and replace Hardie planks on your own. Fiber cement is heavy, the dust is silica and not something you want in your lungs, and getting the fastening depth right takes a calibrated tool and some practice. A flipped or overdriven nail creates a failure point in the new plank before the day is even over. If the siding on your new detached ADU is failing, call (559) 254-0116 and we’ll come look at it. The diagnostic visit is the start of a written quote, not a sales pitch.
FAQs
James Hardie siding carries a 30-year limited transferable product warranty, and properly installed with the right clearances and flashing, it should last 40 – 50 years in our climate. The failures we see are nearly always installation failures that appear in the first five years, not material failures from age. Call (559) 254-0116 if your siding is failing early; we’ll tell you whether the product or the install is at fault.
Small nail pops and hairline stress cracks can sometimes be remediated with proper re-fastening and a color-matched patch product, but any plank with a through-crack, delamination, or edge wicking needs to come out. Patching a plank that’s already compromised buys you a season or two and costs more over time. Call (559) 254-0116 and we’ll give you a straight answer on which planks can stay.
A single elevation replacement typically takes two to three working days, including flashing corrections and paint. A full re-side of a detached ADU takes five to eight working days depending on weather, the amount of sheathing repair, and whether you’re matching an existing factory color. We schedule the work in writing and hit the dates we commit to.
Yes, we keep the common profiles and trim stock for HardiePlank lap, HardiePanel vertical, and HardieTrim on our vehicles running Woodland Hills and the West Valley. If a color needs to be matched from a factory order, we can get it within a week through our suppliers. Call (559) 254-0116 to confirm stock for your specific profile.
If your James Hardie siding on a new detached ADU is cracking, wicking, or pulling away at the corners, call Fielding ADU Woodland Hills at (559) 254-0116. Reed Calloway and the crew will look at it, put the scope and price in writing, and build it right. Scope in writing. Price in writing. Then we build.
Written by Reed Calloway, Owner, Fielding ADU Woodland Hills at Fielding ADU Woodland Hills, serving Woodland Hills and nearby areas.
