Guide
What homeowners need to know.
Short Answer
A Bay Area ADU is usually a six-figure project once design, permits, site work, utilities, and construction are included. Detached ADUs often cost more than garage conversions because they require more site work and building envelope scope.
What Drives Cost
The biggest cost drivers are ADU type, square footage, foundation, utilities, fire access, structural work, finish level, city comments, and whether drawings are ready. The right first move is a feasibility review, not a blind contractor bid.
Budget Planning Rule
Use ranges for early planning, then move quickly into drawings and assumptions. A single attractive number before scope is defined is not a budget; it is bait.
Cost table
Use ranges until scope is real.
| Item | Planning range | Why it moves |
|---|---|---|
| Garage conversion | Lower to mid six figures | Depends heavily on slab, utilities, fire separation, and existing conditions. |
| Detached ADU | Mid to high six figures | Foundation, utility trenching, envelope, and finishes can move cost quickly. |
| Soft costs | Variable | Drawings, engineering, plan check, fees, surveys, and consultants. |
Mistakes
Avoid these expensive shortcuts.
- Shopping only construction bids before drawings exist
- Forgetting utility assumptions
- Comparing prefab, garage conversion, and custom detached ADUs as if they are the same thing
FAQ
Fast answers.
Is a garage conversion cheaper than a detached ADU?
Often, but not always. Existing conditions, utilities, fire separation, structure, and code upgrades can erase the apparent savings.
Should I get plans before ADU bids?
Yes if you want bids that mean something. Plans create comparable scope.
What ADU cost gets missed most often?
Utilities, site work, soft costs, and city response time are the usual suspects.