How Much Does It Cost to Build an ADU in the Bay Area? (2026)
As of 2026, a full ADU in the Bay Area runs about $150,000 to $400,000, and a room addition runs about $200 to $500 per square foot. These are published market ranges, not our quote. On the Peninsula, the biggest cost swing is not size. It is whether you build attached or detached, and which city's size and permit rules apply.
How much does an ADU cost in the Bay Area in 2026?
A full ADU in the Bay Area runs about $150,000 to $400,000 as of 2026. A room addition runs about $200 to $500 per square foot. Those numbers come from published 2026 cost guides. They are market ranges, not our quote. We build both kinds of projects through our home additions and ADU service. We do not give a dollar figure until we see your lot and your plans.
| ADU or addition scope | 2026 Bay Area market range |
|---|---|
| Full ADU | $150,000 to $400,000 |
| Room addition | $200 to $500 per square foot |
Source: published 2026 cost guides. Market range, not our quote.
How does that compare to the rest of the country? A national ADU cost guide, published April 23, 2026, puts the national average ADU at about $180,000. Most homeowners across the country spend $40,000 to $360,000, or $150 to $300 per square foot. Costly projects in pricey areas can run past $600 per square foot. The Bay Area range sits at or above that national average. High Peninsula home values are a big reason why. This is a national figure for context, not a Bay Area quote.
Every ADU job we do starts with a free written estimate and a clear plan before any work begins. Call for a free estimate and get a real number for your lot instead of a market range.
What is the cost difference between an attached and a detached ADU?
An attached ADU usually costs less than a detached one. It shares one or more walls with your house, along with the existing foundation and some utility hookups. That cuts the amount of new structure a crew has to build.
A detached ADU stands on its own. It gets its own foundation, roof, and front door. It costs more to build. But it gives you a fully private, separate unit. Many homeowners pick detached when they want a rental unit or a real separate home for family.
A second-story addition is a third path. It builds up instead of out, so it keeps your yard. It needs a strong foundation and structural base. We may add seismic bracing on a second-story addition to keep the home safe. That bracing is common on older Peninsula houses. We match the new roofline and roof to your existing house.
Why do Bay Area ADUs cost more than the national average?
Bay Area ADUs cost more mainly because Peninsula home values and lot values run well above the national average. Here are real numbers from the U.S. Census for the cities we serve:
- In Millbrae, the median home value is about $1.93 million (ACS 2020-2024 5-year data), and about 61 percent of homes are single-family detached.
- In San Francisco, the median home value is about $1.31 million (ACS 2024 1-year data).
- In San Mateo, the median home value is about $1,639,400 (ACS 2024 1-year data).
- In South San Francisco, the median home value is about $1.19 million (ACS 2020-2024 5-year data).
High home values change the math two ways. First, they support bigger, better-built projects. A well-built ADU protects a large asset. Second, they make ADU rental income or extra family space genuinely worth the cost. Millbrae has gone a step further. It built its own pre-approved ADU plan checklist through its Building Division. That checklist can shorten the design and permit timeline compared to a plan review that starts from scratch.
How do San Francisco's ADU size limits change the cost?
San Francisco has allowed ADUs citywide since 2016. That covers every part of the city zoned for homes. The city permits converted, attached, detached, and junior ADUs. A studio or 1-bedroom ADU is capped at 850 square feet. A 2-bedroom or larger unit is capped at 1,000 square feet.
That size cap limits the top end of your build cost inside the city. A bigger detached ADU might fit on a larger Peninsula lot elsewhere. That same size is not always allowed in San Francisco. Because of that, an estimate for a San Francisco property has to plan around San Francisco's citywide ADU size limits from the first visit, not after the plans are drawn. Every homeowner in a served city still follows the same California ADU approval process. State law sets the floor every city's review must follow.
What drives the price of an ADU up or down?
Six real factors move an ADU price more than anything else. We quote against all of them on every estimate.
Size and how many rooms you add. A small studio ADU costs less to finish than a larger unit with a separate bedroom.
Foundation work and whether you build a second story. New foundations and structural work for a second story add real cost.
New plumbing, electrical, and utility hookups. A full kitchen and bathroom need their own supply lines, drains, and circuits.
Attached or detached. An attached unit shares your home's foundation and some utilities. A detached unit needs its own from the ground up.
Finishes, cabinets, and fixtures. Your choices here can swing the budget as much as the size of the unit.
Lot access and site prep. A tight lot or hard-to-reach backyard adds time and cost before framing even starts.
We are a licensed California general contractor, CSLB License #1085926. We pull the permits your ADU needs and manage the inspections. That keeps the work legal and up to code, and it keeps the paperwork on us, not on you. For a whole-home project, see our full remodeling and construction team.
Ready to see your real number instead of a market range? Contact Angel Suarez Construction for a free written estimate. We are a licensed California B General Building contractor, CSLB License #1085926, founded in 2021 and based in San Bruno. As a service-area business, we come to you across the San Francisco Peninsula, including San Bruno, South San Francisco, San Mateo, Burlingame, Millbrae, San Francisco, Pacifica, Foster City, Belmont, and San Carlos. We give you a clear plan and an honest price in writing before any work starts.
Talk to Angel Suarez Construction
Questions about your general contractor job? We serve San Bruno and the surrounding area with honest, upfront advice.
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FAQ
Related questions
Is a garage conversion cheaper than a detached ADU?
Yes, in most cases. A garage conversion reuses an existing foundation, walls, and roof, so it usually costs less than building a detached ADU from the ground up. A detached ADU needs a brand-new foundation and full structure, which adds cost but gives you a completely separate building. Your real number depends on your garage's condition and your lot.
Does building an ADU increase my home's value on the Peninsula?
An ADU adds real square footage, a rentable unit, or space for family. Peninsula home values are already high too. Medians run from about $1.19 million in South San Francisco to $1.93 million in Millbrae. A well-built ADU protects and can grow that value. We do not give a resale number. Ask a local appraiser or agent for your specific home.
What does ASC's free ADU estimate include?
Every ADU estimate starts with a free visit to your home. We measure your lot, check setbacks and access, and give you a written plan and price before any work begins. We also draw the plans and handle the city approvals for you, so you are not stuck doing the paperwork yourself.