Guide
What homeowners need to know.
Start with ADU type
Detached new-build ADUs, garage conversions, prefab ADUs, attached ADUs, and JADUs do not price the same way. Sorting the project type first keeps the estimate from becoming theatre.
Plans and permits are not optional fluff
Most serious ADU paths require drawings, engineering or energy documentation, city submittal, corrections, and inspections. Treat that as part of the project, not paperwork afterthought.
Site work is the budget swing
Foundation, grading, trenching, sewer, water, electrical, gas removal, fire access, crane access, and drainage can move the number more than cabinet style.
Compare total project assumptions
A prefab package, design-build proposal, and contractor bid may include different scopes. Compare inclusions before comparing prices.
Cost table
Use ranges until scope is real.
| Item | Planning range | Why it moves |
|---|---|---|
| Detached custom ADU | $300k-$600k+ | Driven by size, foundation, utilities, finish level, site access, and city review. |
| Garage conversion ADU | $120k-$300k+ | Can be lower if the structure is sound; upgrades can make it less cheap than it looks. |
| Prefab ADU installed path | $250k-$500k+ | Unit price is not total project cost. Site work, utility hookups, and permits still matter. |
| Plans, engineering, permit support | $12k-$45k+ | Varies by scope, city comments, engineering, Title 24, survey, and corrections. |
Mistakes
Avoid these expensive shortcuts.
- Comparing unit price to all-in project price.
- Ignoring utilities until after drawings.
- Pricing a garage conversion before checking slab, roof, fire, and moisture conditions.
- Assuming a city will approve a generic plan without site-specific work.
FAQ
Fast answers.
What is the biggest ADU cost driver in the Bay Area?
Site conditions and utilities are often the biggest unknowns after ADU type and size.
Is a garage conversion the cheapest ADU?
Often, but not always. Existing structures can hide expensive code, moisture, fire, and structural fixes.
Sources