Measure room by room or slab section by slab section, get one total, and see what that area tends to cost across the scopes we build in the Bay Area.
Enter each room, slab, or section separately. Length times width, in feet.
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.
Foundation square footage is not the same number as living square footage, and mixing the two is the fastest way to blow a budget. A foundation or slab is measured to the outside face of the concrete, so it includes the wall thickness that interior room measurements leave out. On a 1,200 sq ft house that gap is commonly 40 to 70 sq ft. Meanwhile covered porches, cantilevered floors, and roof overhangs add roof area without adding slab, so they cut the other way. When you are pricing concrete, measure the ground the concrete will actually cover.
Break odd shapes into rectangles and add them, which is exactly what the rows above are for. An L-shaped addition is two rectangles. A driveway with an apron and a side walkway is three. For a circular or curved patio, take the widest length and width and accept that you are running a few percent high — overestimating flatwork area is far cheaper than running short on a pour day.
The four ranges shown in the results are separate scopes of work, not stages you add together. A foundation-only number covers excavation, forms, rebar, concrete, and inspection with nothing above it. A full ADU build number is everything from that same slab up through framing, roofing, windows, mechanical, electrical, plumbing, finishes, and utility connections, which is why it lands ten times higher per foot. An addition runs higher still per square foot than an ADU, because you are cutting into an existing house, matching existing framing and finishes, and working around a family that still lives there. Flatwork is the cheapest per foot because it is a few inches of concrete on compacted base with no structure above it.
Bay Area square-foot costs also move with jurisdiction. Permit and school-impact fees, geotechnical requirements, stormwater and impervious-surface rules, and inspection cycles all vary between Santa Clara County, Alameda County, and San Mateo County cities. Two identical 800 sq ft ADUs a few miles apart can differ meaningfully on soft costs alone. Use the area from this tool as the input to a real site walk rather than as a budget by itself.
For concrete, measure the outside face of the foundation or slab. Interior room dimensions leave out wall thickness and will under-count your pour by roughly 3 to 6 percent on a typical house.
No. The foundation-only range is already included inside the full build range. They are alternative scopes: pick the one that matches what you are actually buying.
Split it into rectangles and enter each one as its own row. The tool adds them for you and shows the breakdown so you can check your own math.
For a concrete scope, yes, any slab you are pouring counts. For a living-area or ADU size limit, garages and covered patios are usually treated differently, so confirm how your city counts them before you design to a number.
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.
(925) 592-7162
ProHomeFoundation