Build cost lab · SF Bay Area
2026 Bay Area Prefab ADU Index
What an installed prefab ADU costs city by city, how long each city takes to approve one, and the rules that shape what you can build — compiled from city ordinances, published programs, and current market data. Updated July 2026.
Installed prefab ADU cost by city
Complete installed cost per unit — factory price plus foundation, site work, and installation. Ranges reflect size (roughly 250–1,200 sq ft), finish level, and site conditions. These are city-level market ranges, not estimates for a specific property: slope, utility distance, site access, and permit fees move an individual project outside them.
| City | County | Low | Median | High |
|---|---|---|---|---|
| Danville | Contra Costa | $150,000 | $300,000 | $500,000 |
| Fremont | Alameda | $200,000 | $300,000 | $400,000 |
| Los Gatos | Santa Clara | $150,000 | $250,000 | $350,000 |
| Menlo Park | San Mateo | $150,000 | $250,000 | $400,000 |
| Millbrae | San Mateo | $150,000 | $250,000 | $350,000 |
| Orinda | Contra Costa | $150,000 | $250,000 | $400,000 |
| San Carlos | San Mateo | $200,000 | $280,000 | $400,000 |
| San Mateo | San Mateo | $150,000 | $250,000 | $400,000 |
Burlingame data is published per square foot rather than per unit: roughly $250–$350/sq ft for a prefab ADU before land costs. Cities we cover whose cost data didn't meet our verification bar (listing prices for unrelated product types, factory-only base prices, or financing figures rather than build costs) are excluded rather than estimated — see methodology.
Permit approval timelines by city
Typical time from complete application to permit in hand. California law requires cities to act on complete ADU applications within 60 days, but real-world timelines — including plan-check rounds and corrections — vary widely.
| City | Typical approval timeline |
|---|---|
| Pleasanton | ~6 weeks (42 days) |
| San Ramon | ~6 weeks (42 days) |
| Cupertino | ~2 months (60 days) |
| Pacifica | ~2 months (60 days) |
| Burlingame | ~3 months (90 days) |
| Fremont | ~3 months (90 days) |
| Los Altos | ~3 months (90 days) |
| Los Gatos | ~3 months (90 days) |
| Menlo Park | ~3 months (90 days) |
| Foster City | ~4 months (120 days) |
| Millbrae | ~6 months (180 days) |
| San Carlos | ~6 months (180 days) |
The spread is the story: an identical prefab unit can be approved in six weeks in Pleasanton or San Ramon and take six months in Millbrae or San Carlos — before a single day of site work.
ADU rules snapshot
The local rules that most affect prefab ADU planning, for cities where we verified specifics. State law sets the floor (ministerial approval, at least 800 sq ft allowed, 4-ft side/rear setbacks); cities differ on caps, heights, and extras.
| City | Detached max size | Height limit | Setbacks | Parking |
|---|---|---|---|---|
| Burlingame | — | 16 ft; 18 ft near transit/multifamily; 20 ft matching roof pitch | 4 ft side/rear; 4 ft from other structures | — |
| Fremont | 1,200 sq ft (JADU 500) | 16 ft; up to 18–20 ft near transit or matching roof pitch | 4 ft side/rear; none for conversions | None required |
| Los Altos | 1,200 sq ft (min 150) | 16 ft; 18 ft multifamily/near transit; 20 ft matching roof pitch | 4 ft side/rear; none for conversions | None required |
| Los Gatos | 1,200 sq ft | 16 ft; up to 18–20 ft near transit; attached up to 25 ft | 4 ft side/rear; 5 ft between structures | — |
| Pleasanton | 150–1,200 sq ft | 18 ft | State standard | — |
| San Ramon | 150–1,200 sq ft | 18 ft | State standard | — |
Two programs worth knowing about
- Pleasanton waives development impact fees for ADUs under 750 sq ft — on a Bay Area build, that class of fees can run well into five figures.
- Los Altos runs a Pre-Approved ADU Program with free permit-ready plans, cutting both design cost and plan-check time.
Methodology
Figures come from the research behind our city guides: city ADU ordinances and municipal codes, published city programs, California state ADU law, and current prefab/modular market pricing for each city. Costs are installed per-unit ranges (factory + foundation + site + install), not factory-only quotes. Cities are excluded from a table when the available data didn't meet our bar — we found financing figures, unrelated listing types, or factory-base prices instead of installed costs for several cities, and we exclude rather than estimate. Timelines are typical complete-application-to-permit durations; individual projects vary with plan-check corrections and workload. Full sourcing rules: editorial policy.
License: This dataset is published under CC BY 4.0 — cite "Prefab Cost Lab, 2026 Bay Area Prefab ADU Index" with a link to this page and use it freely.
City guides
Alameda County
Marin County