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CGI Home Additions & ADU: The San Francisco Owner’s Guide
A full home addition or ADU in San Francisco typically runs $53,100 to $153,400 depending on whether you’re adding a single room or building a detached unit from the foundation up. We install CGI-compatible framing, exterior sheathing, and finish materials on additions and ADUs across the city, and we’ve done enough of them to tell you the truth about what holds up in this climate. Most owners don’t discover a problem until fog-season moisture shows up as a swollen exterior corner or a window that won’t square in its opening.
What we see most often is not a single failed part but a sequence: one undersized beam deflects, the framing pulls, and then every finish downstream starts to complain. The good news is the fix usually starts with one correct diagnosis, a written price, and a crew that shows up when we said we would.
Which CGI Models This Applies To
CGI home additions and ADUs in San Francisco are not a one-size-fits-all situation. The framing, sheathing, and exterior systems on an attached addition behave differently than those on a detached ADU in a backyard. An attached addition shares a wall with the existing house; a detached ADU carries its own envelope on all four sides. The load path, the drainage plane, and the tie-down details all change.
The easiest way to identify what you have is to check the permit drawings from the original build. The structural pages will list the beam sizes, post locations, and shear wall panels. If the drawings are gone, we measure from the inside: joist spacing, stud width, and the thickness of the exterior sheathing tell us most of what we need to know before we open a single wall. When OEM components are still available, we use them. When they’re not, we specify aftermarket equivalents that meet or exceed the original spec, and we put that substitution in writing before any work begins.
One CGI-specific gotcha we keep running into: exterior window and door rough openings on San Francisco additions built before the 2013 code cycle often come in under the current minimums for egress and header depth. It’s not a failure until you try to replace a window or door, and then the opening has to be reframed. We check for that on every assessment.
Styles & brands we work with
Signs Your CGI Home Additions & ADU Is Failing
- Doors that stick in the rainy season. Fog and rain swell the framing and the door slab. If it sticks every winter and frees up every summer, the rough opening is moving. That points to moisture getting where it shouldn’t be, not a bad door.
- A hairline crack running from a window corner to the ceiling. This is usually shear movement in the wall. Small cracks are common; a crack that grows a millimeter every month is not. We’ve opened walls in the Outer Sunset and found the sheathing nails pulled right out of the studs.
- Floor bounce in one specific spot. A joist that deflects under foot means the span was overbuilt or a support has settled. If you can feel the floor move when someone walks past, don’t wait. The finish materials on top are taking damage every day.
- Sagging or uneven exterior trim. When the trim line dips, the wall behind it has shifted. On additions, this often traces back to an undersized header over a wide window or door opening.
- Mold or musty odor in one corner. San Francisco’s marine air pushes moisture into north-facing walls all summer. If the building paper was installed wrong or has degraded, you’ll smell it before you see it.
See the difference
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AfterReplacement Cost & What’s Included
Home additions in San Francisco run $53,100 to $153,400; ADU construction runs $106,200 to $259,600; and garage conversions sit between $35,400 and $94,400. The spread comes down to scope: a single room bump-out with simple finishes lands at the low end, a full detached ADU with its own kitchen, bath, and utility connections lands at the top. Size, site access, structural modifications, and finish level all move the number.
Every project gets a written price before work starts. That price includes the parts, the labor, the permits, and the haul-away. It also includes our 365-Day Done Right Promise: if the work fails inside the first year, we come back and correct it at no charge. No numbers that move at the door. No surprise change orders halfway through. Curtis Lane signs the scope document, and the guarantee is in your hand at close-out.
DIY or Call Us?
Here’s the honest split. You can safely check visible trim, look for cracks, and watch whether doors stick seasonally. What you should not do is open a load-bearing wall or try to shore up a settling corner yourself. A wrongly placed jack post can knock a wall out of plumb faster than you can react. Structural work on an addition or ADU needs permit drawings, engineering review where required, and an inspector’s sign-off. That’s not gatekeeping; that’s the California Building Code.
If you can see movement, water staining, or separation between the addition and the main house, call us at (831) 610-5598. We’ll walk the site, give you a straight number, and tell you what actually needs to happen. Straight number, real scope, no surprises at close-out.
FAQs
A properly built addition or ADU in San Francisco should last 30 to 50 years before major structural intervention, with exterior siding and trim needing replacement at the 15 to 25 year mark. Fog-driven moisture is the main aging factor here; a north-facing wall takes more weather than the rest of the structure combined. If you’re seeing structural issues inside 10 years, something was missed at the time of construction. Call (831) 610-5598 and we’ll find it.
Most failures can be repaired without rebuilding, as long as the foundation and the main load path are sound. We’ve corrected deflecting beams, re-sheathed wet walls, and re-squared window openings without tearing down a room. The cases that require a rebuild are usually foundation settlement or rot that has spread through more than 30 percent of the wall framing. We’ll tell you which one you have before we bid it.
A targeted structural repair, like jacking a beam or re-sheathing a section of wall, usually takes one to three working days once permits clear. Full additions and ADU builds run from three to six months depending on site conditions and finish selections. Permitting in San Francisco is its own timeline; we pull the permits and shepherd them through the building department so you don’t have to chase anything.
We stock common structural components on the truck: engineered lumber in standard sizes, structural screws, hold-downs, and sheathing in the grades CGI specifications call for. For model-specific finish materials or exterior products, we order direct from our documented supplier list, including KraftMaid, Cambria, Silestone, and other brands we specify for additions and ADUs in San Francisco. Call (831) 610-5598 for an exact quote; estimates are free.
Written by Curtis Lane, Owner, Bradford Remodeling Co – San Francisco, CA at Bradford Remodeling Co San Francisco, serving San Francisco and nearby areas.
