CWS Home Additions & ADU Nationwide

CWS Home Additions & ADU: The Cupertino Owner’s Guide

We stock and install CWS-compatible structural components and finishing systems for home additions and ADUs in Cupertino, with typical installed project costs ranging from $45,000 to $130,000 for additions and $90,000 to $220,000 for detached ADUs. Most CWS material failures we see in Cupertino involve water-intrusion damage to the weep screed drainage plane, thermal expansion stress on the stucco membrane, or subfloor settling in the Monta Vista area’s clay-heavy soil. A written price with named brands and line-item costs lands before any work starts, per Clause 1 of the Haven Standard.

Call (669) 202-9618

Homeowners usually spot the problem first as hairline cracks propagating from window corners, a soft spot in the addition’s subfloor, or water staining where the new roof ties into the existing structure. In Cupertino’s Mediterranean climate, those cracks widen through October’s first rains, and the subfloor issue gets worse as the clay soil swells and contracts through winter.

Which CWS Models This Applies To

CWS, or California Window Systems, manufactures structural framing components, window-wall systems, and waterproofing membranes used in home additions and ADU construction across Northern California. We work with CWS Series 500 and Series 700 thermal break aluminum framing, CWS-90 sliding door systems, and their proprietary stucco-integrated weep screed assemblies. The Series 700 carries a deeper thermal break, 1.5 inches versus the Series 500’s 0.75 inch, which matters in Cupertino’s temperature swings between 40°F January mornings and 85°F September afternoons.

To identify your CWS component, check the etched marking on the aluminum extrusion’s interior face, typically within six inches of the corner joint. The label reads “CWS-[series]-[year]” followed by a batch code. For the weep screed, the part number is stamped on the galvanized flange that sits behind the stucco base coat. OEM CWS parts ship from their Hayward distribution center with a three-week lead time; we stock compatible aftermarket thermal breaks and weep screed profiles from Pacific Coast Building Products that match CWS specs and carry the same 20-year material warranty. The aftermarket decision comes down to timeline, not quality, in every case we’ve documented.

Signs Your CWS Home Additions & ADU Is Failing

• Diagonal cracking at window corners. This indicates thermal expansion stress on the CWS aluminum frame exceeding the stucco membrane’s flexibility. In Cupertino’s climate, the daily 30-degree swing between shade and full sun on a south-facing addition puts 2,000-plus expansion cycles per year on the frame.
• Soft or bouncy subfloor near the addition’s perimeter. The clay soils in Monta Vista and Rancho Rinconada expand when wet and shrink when dry, creating differential settlement where the original foundation meets the new stem wall. The CWS sill plate compresses or the anchor bolts loosen, and the subfloor span exceeds its design load.
• Water staining on interior drywall below the roof tie-in. The CWS-compatible flashing membrane at the addition’s roof junction has either delaminated or was installed without the required 6-inch overlap onto the existing roof deck. Cupertino’s winter wind-driven rain finds that gap reliably.
• Condensation between glass panes on CWS-90 sliding doors. The insulated glass unit’s edge seal has failed, breaking the argon fill. This is a glass replacement, not a frame replacement, but owners often misdiagnose it as a structural issue.
• Efflorescence on exterior stucco below grade. The weep screed’s drainage slots are clogged with stucco overspray or debris, trapping moisture in the wall cavity. In Cupertino’s hard-water areas near Stevens Creek, the mineral deposit buildup accelerates once the drainage path is blocked.

Before

After

Replacement Cost & What’s Included

What drives the range: accessibility (second-story additions cost more to reach), stucco texture matching complexity, and whether the damage has propagated into the structural framing. Every quote names the brand, the spec, and the labor hours. No work proceeds without a signed change order if the scope shifts, which is how we’ve kept 4,000-plus projects within a few percent of the written price.

The Haven Standard 365-Day Done Right Promise covers all CWS component work. The documented photo record, included as standard, shows the condition before, the part installed, and the drainage test or thermal verification.

DIY or Call Us?

Here’s what you can safely check: verify the CWS part number from the extrusion marking, photograph the crack pattern or water stain for reference, and confirm whether your addition or ADU was permitted through the City of Cupertino’s building department. Permits pulled after 2019 for ADUs in Cupertino require CWS or equivalent thermal performance, and the documentation helps us match the spec exactly.

Where DIY goes wrong: the weep screed replacement requires cutting stucco without damaging the water-resistive barrier behind it, a two-person operation with specialized grinding equipment. Thermal break frame replacement demands torque-calibrated fastening to avoid deforming the extrusion. And any work on the roof tie-in voids the existing roofing warranty if not documented with manufacturer-approved materials. We’ve rebuilt three Cupertino additions this year where the owner’s waterproofing attempt trapped more moisture than it stopped.

Measure twice, document once, explain it plainly. Call (669) 202-9618 and we’ll schedule a free estimate with the written price and photo record included.

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