How this calculator works
If the ADU is 750 sq ft or less (JADU 500 or less): no impact fee. Otherwise: proportional fee = city fee × (ADU area ÷ main house area), capped at the full fee. Your city may compute proportionality differently.
What each input means
| Input | What it is actually asking |
|---|---|
| Interior livable area | Measured inside the walls. Ask your city how it measures. |
| City fee | The impact fee your city lists. We publish no city fees. |
Worked examples
Including one where the naive answer misleads, which is the example most calculators leave out.
A 749 sq ft unit
Type 749. The tool says impact fees do not apply. That is why many plans stop just under 750.
An 800 sq ft unit
Type 800, a 1,600 sq ft house and your city’s fee. The tool shows half the fee on a straight ratio.
What changes the result
The fee moves from none to proportional at the 750 sq ft line. Below 500 sq ft, school fee treatment also changes.
When not to use this
Do not use this for connection or capacity charges, plan check fees or permit fees; they are separate. The ADU fee estimator covers the wider fee picture.
Questions this calculator answers
- Are ADUs exempt from impact fees in California?
- At 750 sq ft or less of interior livable space, yes.
- Do JADUs pay impact fees?
- Not at 500 sq ft or less.
- What about water and sewer charges?
- Connection and capacity charges are a separate rule and may still apply.
- If my fees look wrong, what do I do?
- Ask for an itemized list. See ADU impact fees too high.
Sources and methodology
Figures dated 27 October 2026. Last reviewed .
- Gov. Code § 66311.5 — impact fees, connection fees and capacity charges (California Legislature, retrieved 2026-09-05. Renumbered from § 66324 by SB 543, effective January 1, 2026.)
Related
- ADU fee estimator The 750 sq ft cliff on your city’s figure.
- ADU square footage calculator Check your area.
- California ADU fees by city Published fee research.