CGI Home Additions & ADU Nationwide

CGI Home Additions & ADU: The Redwood City Owner’s Guide

CGI home additions and ADU components in Redwood City typically run $45,000-$130,000 installed, depending on whether you’re extending an existing structure or building a detached unit from the slab up. We stock CGI-compatible framing hardware, window systems, and exterior finishes for same-day assessment, and we carry KraftMaid and Cambria lines for the interior fit-out. Most Redwood City owners first notice trouble when a CGI addition starts showing water intrusion at the tie-in point, or when the ADU’s prefab panel seams gap during our dry summers and wet winters.

Call (650) 560-7228

Which CGI Models This Applies To

CGI builds three main product tiers: the Classic Series (stick-framed additions with site-built tie-ins), the Precision Series (panelized wall systems with factory-sealed joints), and the Haven Series (modular ADU units delivered in 8-14 foot sections). The Classic uses standard dimensional lumber with CGI’s proprietary header brackets - you’ll find the model stamp on a metal tag inside the utility chase, usually near the electrical panel. Precision units carry a QR code on the exterior sheathing, typically behind the downspout on the street-facing wall. Haven modular units have a serial plate on the mechanical closet door.

For replacement parts, the Classic Series accepts standard Simpson Strong-Tie hardware with no modification. Precision panels require CGI’s own gasket-and-track system; aftermarket gaskets exist from Tremco and Dow, but the interlock geometry differs by 3/16 inch, which is enough to break the rainscreen in a driving Peninsula storm. Haven modular units need factory-ordered splice plates for any structural connection. We keep Classic hardware and Precision gaskets on our Redwood City truck; Haven plates ship from CGI’s Sacramento warehouse in 48 hours.

We’re an independent service provider - not affiliated with CGI - and we source through the same distributors that supply their warranty crews.

Signs Your CGI Home Additions & ADU Is Failing

• Water staining at the original wall tie-in. The flashing detail between your existing home and the CGI addition carries lateral movement that caulk alone can’t absorb. In Redwood City’s clay-heavy soils, seasonal settling cracks the sealant bead and water tracks along the top plate.
• Panel seams visible from the curb. Precision Series gaskets compress permanently after 8-12 years of UV exposure. Once you can see daylight through a seam, the rainscreen is compromised and the OSB sheathing behind it is taking moisture.
• Doors or windows that stick seasonally. CGI units use engineered headers that perform best within a 1/4-inch tolerance. If the foundation has settled or the sill plate has twisted - common in the Stambaugh-Heller neighborhood’s older lots with irregular drainage - the frame goes out of square and binds the operable units.
• Musty odor in the mechanical chase. Haven Series units route HVAC and plumbing through a central chase that shares walls with the exterior envelope. A failed chase seal lets attic air into the conditioned space, and Redwood City’s marine layer humidity feeds mold in 72 hours.
• Exterior trim pulling away at corners. CGI’s PVC trim expands at 3.5 times the rate of the fiber-cement siding it butts against. Without the correct slip-joint detail, the trim buckles and opens a path for carpenter ants, which we see regularly in the hills above Farm Hill Boulevard.

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Replacement Cost & What’s Included

The wide range on full additions reflects three variables: whether we’re tying into slab-on-grade or raised foundation, whether Redwood City requires a full fire separation for the ADU, and how far we need to extend electrical service from the main panel. Every quote is flat-priced with a written scope before work starts - no call-for-pricing, no allowance games. Under Clause 1 of The Haven Standard, the number you sign is the number you pay, and any change needs your signature before we touch a tool.

DIY or Call Us?

There’s a safe middle ground here. A homeowner can walk the perimeter with a flashlight, look for the symptoms above, and photograph anything suspicious - that documentation helps us enormously and costs you nothing. You can also check the CGI model tag and confirm whether your unit is still under factory warranty; CGI covers gasket defects for 10 years and structural connections for 20, but they require annual inspection records that most owners don’t keep.

Where DIY goes wrong is the tie-in work. The flashing between your existing home and the CGI addition is a life-safety detail: if water gets in and rots the sill plate, you’re looking at structural repair, not maintenance. The panel gaskets on Precision units are under compression tension when installed; removing them without supporting the panel above can distort the frame. And any electrical extension to service an ADU requires a Redwood City permit and inspection - unpermitted work kills resale value and insurance coverage.

We don’t charge to look. Call (650) 560-7228, we’ll schedule a documented site visit, and you’ll get a written price before any work starts.

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