ADU Cost in the Bay Area: 2026 Price Ranges by Size and Site

Start with a planning number, then adjust it for your lot. Alameda County's ADU Resource Center offers a very rough placeholder of $400 to $550 per square foot for construction plus design and fees. The Association of Bay Area Governments (ABAG) uses the same $400 to $550 rule of thumb in its January 2025 ADU Work Group materials and breaks it down by unit type in the table below.

If you're researching the cost to build an ADU in California in 2026, you'll also see statewide numbers, and they land in the same neighborhood. Casita Coalition, a statewide ADU nonprofit, says most projects fall between $250 and $500 per square foot and calls $400 a good starting estimate. A UC Berkeley survey of ADU owners found a Bay Area median construction cost of about $329 per square foot for ADUs of all types permitted or finished in 2018 and 2019. Treat that as history, not a budget. Labor and materials have gone up since.

Look at the table again: the studio costs more per square foot than the two-bedroom. That's normal. A 500 sq ft unit still needs a kitchen, a bathroom, a foundation and utility hookups, and none of those get much cheaper when the footprint shrinks. Alameda County puts kitchen work at about $25,000 to $50,000 and each bathroom at $15,000 to $25,000.

You can see the range of sizes in our own work. Oregon in Berkeley fits a one-bedroom into 400 square feet. Hall in Walnut Creek is a 720 sq ft two-bedroom on a wide, flat lot. Bates is a 710 sq ft one-bedroom on a west-facing El Cerrito hillside. Size is only part of the story, though. The site is the next thing to price.

When you compare quotes, ask what's inside the per-square-foot number. A complete Bay Area ADU budget has three layers.

• Soft costs. Design, engineering, surveys, Title 24 energy calculations and permit fees. ABAG's materials estimate design and permitting at about 15% of costs.
• Hard costs. Foundation, framing, roof, windows and doors, plumbing, electrical, mechanical, insulation, drywall, cabinets and finishes. For construction alone, Alameda County suggests $425 to $500 per square foot for an average project.
• Site and utility work. Grading, retaining walls, tree protection, trenching for water, sewer and power, and electrical panel upgrades. This is often what separates two quotes for the same floor plan.

State law shapes the fee line. HCD's March 2026 ADU Handbook explains that an ADU with 750 square feet or less of interior livable space is exempt from impact fees charged by local agencies, special districts and water corporations. Above 750 sq ft, impact fees have to be proportional to the size of the main house (Gov. Code § 66311.5). School districts can still charge fees on ADUs over 500 square feet. Newly built detached ADUs also generally need solar under the California Energy Code, so make sure your quote covers it.

This page covers what an ADU costs. For how to pay for one, including HELOCs, construction loans and Type Five's financing partner, read our ADU cost and financing guide.

Square footage sets the baseline. Your lot decides how far above it you land. Alameda County lists lot complexity (slope, trees, fault lines), required utility and site upgrades, and sprinkler requirements among the factors that move construction cost.

Slope and soil. Hillside lots in the Berkeley and Oakland hills, Orinda and El Cerrito often need engineered foundations, retaining walls and drainage work that a flat backyard doesn't.

Access. Narrow side yards, steep driveways and long carries from the street slow every delivery and every trade. You pay for that time even when the ADU itself is simple.

Utilities. Alameda County's ADU Guidebook notes that ADUs usually don't need separate water and sewer connections, but some agencies require services or meters to be upsized, and PG&E upgrades or new service add time and cost. Put a long trench or a main panel upgrade in the budget from day one.

Fire zones and trees. Lots in a Wildland-Urban Interface zone need fire-resistant exterior materials. Our Hermosa guesthouse in Orinda and Caldwell ADU in Oakland both sit in WUI zones and use Type Five's fire-resistant material package. Trees matter too: Berkeley applies its Coast Live Oak and Creeks ordinances to ADUs.

No online calculator can see your side yard or your electrical panel. That's why Type Five looks at your actual site before giving you a guaranteed budget for the whole project.

Fees usually aren't the biggest line in an ADU budget, but they change from city to city and they're easy to leave out.

• Impact fees. State law bars them for ADUs at or under 750 sq ft. Palo Alto writes the same exemption into its municipal code for ADUs under 750 sq ft and all JADUs, and charges larger ADUs proportionally.
• School fees. Districts can charge them on ADUs over 500 sq ft, up to a statewide maximum the State Allocation Board raised to $5.38 per square foot in January 2026. Your district's adopted rate may be lower. In Berkeley, Berkeley Unified collects the fee and the city needs proof of payment before it issues the building permit. Palo Alto works the same way with its school districts.
• City permit fees. Berkeley charges a $250 zoning plan check fee and a $500 neighbor noticing fee at building permit submittal, then bases building permit fees on project valuation. Alameda County's guidebook says building permit fees are often a few thousand dollars.
• Utility fees. Water, sewer and PG&E charges come from the utility, not the city, and depend on what your property already has.

If you're on the fence about size, keep this in mind: a 740 sq ft ADU and a 900 sq ft ADU can land in different fee categories. Near the 750 sq ft line, size is a budget decision as much as a floor plan one.

For setbacks, height limits and other local rules, start with the city guides below, our Bay Area ADU builder page or the Berkeley ADU builder page.