ADUs · Cost
What does an ADU actually cost in San Jose?
Not a per-square-foot table. What the money is actually buying, and which parts don't get cheaper when the unit gets smaller.
A good-quality ADU runs about $300,000 to $360,000 built. Simpler work exists below that and larger or more complicated sites go above it. We price per project rather than per square foot, for a reason worth understanding before you compare bids.
Why we won't give you a per-square-foot number
Roughly $13,500 to $18,500 of the cost happens before anyone breaks ground, and almost none of it scales with size. The architect charges the same for a 400 square foot unit as a 700. The surveyor does too. The soils report certainly does — the ground doesn't care how big the building is.
So the effective cost per square foot rises as the unit gets smaller. A flat multiplier gets the small end badly wrong, which is exactly the size range most people ask about first.
| Item | Cost | What it's for |
|---|---|---|
| Architectural plans + structural engineering | $10,000 | Full permit set including Title 24. The architect also submits to the city portal |
| Setback survey | $2,500 | Establishes where the unit can legally sit |
| Setback certification letter | $500 | Confirms to the inspector it was built where the survey said |
| Geotechnical soils report | $4,500–$5,500 | Baseline, no geohazard. Determines the foundation design |
| — with liquefaction risk | $8,000–$10,000 | Deeper testing. Real in parts of Santa Clara County |
| Deep-foundation special inspection | ~$762 | Only if the soils report calls for a deep or pile foundation |
| Permits, impact and school fees | Varies by city | Pulled per job — we won't publish a number we'd have to guess |
Two state rules that can move your number
Impact fees are waived under 750 square feet
Under California law, ADUs under 750 square feet are exempt from local impact fees, and JADUs under 500 square feet are as well. On some jurisdictions that is a meaningful saving, and it is a genuine reason to think carefully before designing a 780 square foot unit.
Your city must offer pre-approved plans
Since January 2025, every California city and county has to run a pre-approved ADU plan program and rule on a qualifying application within 30 days. Using one can take the $10,000 plan cost off your project and collapse the permit timeline. How AB 1332 works →
What actually swings the construction number
- Utility runs. Sewer, water, gas and electrical from the house or the street. A unit at the back of a deep lot costs meaningfully more than one near existing service.
- Panel capacity. If the existing panel can't carry the new load, a service upgrade is real work with a utility coordination timeline attached.
- Access. Whether a machine can reach the build area or whether material and spoil move through a side gate by hand.
- Soils and slope. A flat lot and a graded hillside are different foundations at different prices.
- Detached vs garage conversion. A conversion reuses an existing shell, which usually costs less — but often needs structural and envelope upgrades to become legal living space.
How to compare two ADU bids
The gap between bids is usually soft costs one included and the other left out. The four questions to ask both builders →
Common questions
How much does an ADU cost in San Jose in 2026?
Why won't you quote a price per square foot for an ADU?
Are ADUs exempt from impact fees in California?
What does an ADU soils report cost?
Is a garage conversion cheaper than a detached ADU?
Where to go next
Written by the team at Elusive Construction, a licensed California general contractor (B #1112902) building in Santa Clara and San Mateo counties. General information about how residential construction works — not legal advice. For your specific contract, talk to an attorney.
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