Detached, attached, or garage conversion — get a Bay Area planning range for the slab, raised floor, or pier-and-beam foundation plus utility trenching.
Every result is a planning estimate, not a quote or an engineering determination. Real pricing depends on soil conditions, access, existing structure, engineering requirements, and your jurisdiction’s current fee schedule. For structural questions we coordinate with a licensed engineer — we do not perform engineering ourselves. Get a site-specific estimate before committing to a budget.
An ADU foundation is a small pour with all the same fixed costs as a big one. You still mobilize an excavator, still set forms, still order a truck, still schedule an inspection. Spread across 600 sq ft instead of 2,400, those fixed costs push the per-square-foot rate above what you would pay on a main house. That is normal and it is why ADU foundation bids often look expensive on a unit basis.
The three ADU types diverge quickly:
A conversion looks like the cheap path because the concrete is already there. In practice, older Bay Area garage slabs were poured for parking a car, not for living space. Common findings are a slab that is too thin, no perimeter footing of habitable depth, no vapor barrier under it, cracking from soil movement, and a floor pitched toward the old garage door. Any of those can mean saw-cutting and pouring a new perimeter footing, underpinning the existing edge, or building an insulated raised floor over the slab.
None of that is knowable from a photo. We evaluate the existing slab in person, and where the structural condition is in question we coordinate with a licensed engineer for the determination — we build to their stamped detail rather than making the call ourselves. Plan for the possibility in your budget rather than being surprised by it mid-permit.
Trenching is the other line people forget. Water, sewer, and electrical have to reach the ADU, and Bay Area trenching runs roughly $38 to $65 per linear foot depending on what the trench crosses. Cutting through an existing driveway or a mature root zone lands at the high end. If you are weighing options, our ADU foundation page walks through the scope in detail, and framing can often be sequenced with the same crew to save a mobilization.
Sometimes, and it is the single biggest cost saver when it works. Older garage slabs were poured for vehicles, so they are often thin, uninsulated, without a vapor barrier, and lacking a habitable-depth perimeter footing. The slab has to be evaluated in person, and where the structural condition is in question we coordinate with a licensed engineer for the determination.
Underpinning extends or deepens an existing foundation so it can carry new load or match the depth of new construction. It comes up on garage conversions with shallow footings and on attached ADUs where the new foundation ties into the old one. It is hand-dug work in sections, so it adds both cost and schedule.
No. The per-foot figure covers excavation, bedding, backfill, and compaction for the trench. Meter upgrades, panel upgrades, sewer lateral work, and any city connection fees are separate and vary a lot by jurisdiction and by the age of your existing service.
On a typical detached Bay Area ADU the foundation and trenching usually run in the range of 10 to 18 percent of the all-in construction cost. Garage conversions skew lower because much of the concrete already exists. Hillside and poor-soil sites skew higher.
Tell us what you're building — a new home, an ADU, an addition — and we'll get back to you with next steps, usually within one business day.
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