Shape Homes · Journal · Cost

What a modular ADU actually costs in the Bay Area in 2026.

The factory package, the foundation, the trenching and the four line items that decide whether your project lands at $315,000 or $470,000. We walk a real 750 square foot budget from lot check to final inspection.

By the Shape Homes team · Cost · 8 min read · Last updated August 27, 2026

The short answer

A detached, permitted, finished ADU on a flat Bay Area lot lands between $420 and $620 per square foot all-in for 2026 starts. For a 750 square foot one-bedroom that is roughly $315,000 at the clean end and $470,000 when the site fights back. The building is the predictable part. The lot is not.

Where the money goes

1. The factory package

Panels cut from the model, framed on steel jigs, sheathed, taped, weather barrier on and windows set before the truck leaves. This is the number that holds, because it is priced off a drawing set rather than off a crew's day. Window brand is the biggest single lever inside it: moving from a mid-tier aluminium line to Fleetwood or thermally broken steel can swing a 750 square foot unit by $18,000 to $40,000.

2. Foundation and site work

A flat lot with good soil takes a conventional slab. A slope, expansive clay or a geotechnical report calling for piers can triple this line. Grading, drainage, retaining and access for the crane pad all live here. If your driveway cannot take a truck, that is a cost too.

3. Utilities and trenching

The sewer lateral nobody budgets for. Distance from the ADU to the point of connection is the whole story: 30 feet of trench across a lawn is a rounding error, 140 feet through a mature garden with a heritage tree in the path is not. Electrical service upgrades, a new panel, water tap fees and gas or full-electric decisions all land in this bucket.

4. Permits, fees and reports

Plan check, building permit, school fees where they apply, geotechnical and arborist reports, Title 24 energy compliance and, on some parcels, utility connection fees that arrive late and surprise people. State law caps impact fees on units under 750 square feet, which is a real reason many of our ADUs land just under that line.

A real 750 square foot budget

• Factory package, delivered and set — $186,000
• Foundation, grading and drainage — $58,000
• Sewer, water, electrical trenching and connections — $41,000
• Interior finish, cabinetry, appliances and fixtures — $52,000
• Permits, fees, reports and Title 24 — $23,000
• Hardscape, landscape repair and final site finish — $16,000
• Contingency held at 8% — $30,000

That is $406,000, or $541 per square foot, on a Peninsula lot with a 90 foot sewer run and no slope. Take away the trenching distance and it is under $380,000. Add piers and a service upgrade and it clears $460,000.

Why the same building prices differently by town

A flat lot in one city and the same building in another can differ by $60,000 before anyone changes a finish. Review timelines, school and impact fees, fire-hardening requirements, tree ordinances and utility company connection policy all vary. Woodside and Portola Valley add geotechnical and fire scrutiny. Palo Alto and Menlo Park move fast on the ministerial ADU path. Los Altos Hills constrains size before anything else through development-area math.

The four line items that decide your number

• Distance from the unit to the sewer or septic connection
• Slope and soil, which set the foundation type
• Whether your electrical service can carry a second dwelling
• Window and door specification, the largest controllable swing inside the package

How to get a real number

Bring the address. We pull the parcel, your city's ordinance and a street view of the driveway before the call, and we tell you which of the four items above is going to be the expensive one on your lot. That conversation takes twenty minutes and saves the two months people usually spend guessing.

Last updated 2026