South Bay · field-data comparison
ADU cost · permits · timeline · which one wins
South Bay owners deciding between San Jose's pre-approved ADU program (fastest plan-check in the state) and Sunnyvale's straightforward objective-standards review. Lot sizes, hillside overlays, and water-utility splits move the math more than people expect.
| Metric | San Jose | Sunnyvale |
|---|---|---|
Detached ADU ($/sqft) All-in installed, 2026 USD. | $425–$550 | $490–$640 |
Garage conversion ($/sqft) | $250–$360 | $275–$395 |
Ministerial permit clock End-to-end, including agency back-and-forth. | 8–12 wk | 8–12 wk |
Discretionary review | 14–22 wk | 16–26 wk |
Permit story
San Jose has invested heavily in making ADUs easy. The Single Family ADU Process gives by-right approval to ADUs meeting objective standards, and the city's pre-approved plan program can collapse plan check to 4–6 weeks when a standard plan fits the lot. The main schedule variables are the Hillside Combining District on the foothills and which water utility serves the specific block.
Full San Jose field notes →Permit story
Sunnyvale runs ADU plan check through Community Development with a well-documented online portal. State law applies, the city publishes objective design standards, and pre-approved plans accelerate clean projects. Eichler overlays and creek setbacks are the main local wrinkles.
Full Sunnyvale field notes →San Jose
Sunnyvale
San Jose
Sunnyvale
The cheaper detached build, on paper, is San Jose (top-end $550/sqft vs $640/sqft). The faster permit clock belongs to Sunnyvale (12 weeks vs 12 weeks ministerial). Those headlines flip once you factor in overlays, water-utility coordination, and design constraints — which is why we sketch the parcel before we ever quote a number.
Most detached San Jose ADUs land between $285K and $425K all-in. Garage conversions typically run $155K–$245K. Pre-approved plans on a standard lot are the cheapest path.
San Jose maintains a library of architect-designed ADU plans pre-vetted by the city. If your lot fits one, plan check can collapse to 4–6 weeks instead of the typical 8–12. We confirm fit during the first feasibility walk.
Yes — if the ADU meets San Jose's objective standards (setbacks, height, parking exemptions under state law), the city cannot deny it. No public hearing, no discretionary review.
Most detached Sunnyvale ADUs land between $320K and $470K all-in. Garage conversions run $170K–$255K. Eichler-overlay parcels add design and materials cost.
In the Eichler Single-Family Home Combining District, yes — low-pitch roofs, post-and-beam character, and compatible materials are reviewed. Outside those tracts, the requirement does not apply.
Clean ADU projects on flat lots commonly hit 4–7 weeks in plan check. The portal is one of the better ones in the Bay Area for digital submittals.