ADU Fees and Impact Fees
There is a real fee break in state law for smaller units, and it is narrower than the way it usually gets repeated. It applies to impact fees specifically, which is a defined legal term that does not cover the plan check and permit fees most homeowners are actually quoting.
California ADU law was renumbered by SB 477 in March 2024. The sections cited here are the current ones, Gov. Code 66310 to 66342. Local overlays still apply to your parcel, so confirm your specifics with your city building department or San Diego County before finalising plans.
What the rules actually say
- State law: "A local agency, special district, or water corporation shall not impose any impact fee upon the development of an accessory dwelling unit that has 750 square feet of interior livable space or less or a junior accessory dwelling unit that has 500 square feet of interior livable space or less" (Gov. Code 66311.5)
- Above 750 square feet, impact fees "shall be charged proportionately in relation to the square footage of the primary dwelling unit"
- Impact fee is a specific legal term. It does not cover plan check fees, permit fees, utility connection fees, or school fees, all of which can still apply
- So the exemption is real, but "no fees under 750 square feet" is wrong, and you should never plan a budget on that reading
- Your building department publishes the current fee schedule for your jurisdiction. Confirm the total there, not from a rule of thumb
Common questions
Are ADUs exempt from fees in California?
From impact fees, below a size threshold. State law bars any impact fee on an ADU of 750 square feet of interior livable space or less, or a JADU of 500 square feet or less (Gov. Code 66311.5). It does not exempt plan check, permit, utility connection, or school fees, which is why the total on your fee schedule is rarely zero.
What happens above 750 square feet?
Impact fees become chargeable, and the statute requires they be charged proportionately in relation to the square footage of the primary dwelling unit, rather than as a flat amount.
Does the exemption depend on it being a conversion?
No, the threshold is about size, not about how the unit was created. What conversions get separately is the size advantage: converting existing space is not subject to the 1,200 square foot cap that applies to new construction, so a conversion can be sized deliberately against the 750 square foot fee threshold.
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