Short answer
ADU cost depends less on the label and more on utilities, foundation, kitchen and bath density, fire separation, city review, and whether your drawings are ready.
Cost drivers
What moves the number.
Small does not mean cheap
ADUs pack kitchen, bath, utility, and life-safety work into a small footprint, so cost per square foot can be higher than a larger home.
Utility work is the budget trap
Electrical capacity, sewer, water, gas, trenching, and meters can change the estimate quickly.
Plans before bids
Builder quotes are only comparable when everyone prices the same drawings and assumptions.
Planning ranges
Use ranges until scope is real.
| Line item | Planning range | Why it moves |
|---|---|---|
| Detached ADU | $250-$600+/sq ft | Higher in coastal California and complex SF Bay Area lots. |
| Garage conversion | $170-$340+/sq ft | Existing slab, walls, utilities, and code upgrades decide the real spread. |
| Planning and permits | 8%-16%+ | Plans, engineering, energy compliance, plan check, and city fees. |
FAQ
Fast answers.
Is this a contractor bid?
No. It is a planning estimate that helps you understand the likely budget range before drawings, engineering, city review, and contractor pricing.
Why is the range wide?
Early construction budgets should be ranges because site conditions, structural scope, utility work, finishes, permits, and contractor availability can move the number fast.
Do I need to enter contact information?
No. The estimate appears first. Contact information is only for saving the estimate or asking for a local review.
Should I use this before talking to an ADU builder?
Yes. Use it to sanity-check your budget, then get feasibility and permit-ready drawings before treating any builder number as real.
