You price two versions of the same backyard unit. The 740-square-foot plan and the 800-square-foot plan are about $9,000 apart in construction cost — and then the city's fee estimate lands, and the gap is four times that. Nothing about the second unit is harder to serve. It simply crossed a line the Legislature drew, and on the far side of that line a whole category of fees switches on.
The short answer: no local agency, special district or water corporation may impose any impact fee on an ADU with 750 square feet of interior livable space or less, or on a junior ADU with 500 square feet of interior livable space or less. Above 750, impact fees must be charged proportionately in relation to the square footage of the primary dwelling (§ 66311.5).
Key numbers
| Rule | Threshold | Where it comes from |
|---|---|---|
| ADU exempt from impact fees | 750 sq ft of interior livable space or less | § 66311.5(c)(1) |
| JADU exempt from impact fees | 500 sq ft of interior livable space or less | § 66311.5(c)(1) |
| Above the cutoff | Fees charged proportionately to the primary dwelling's square footage | § 66311.5(c)(1) |
| School fees | Not applicable to units under 500 sq ft of interior livable space | § 66311.5(c)(3) |
| Utility connection fees | ADU is not a new residential use unless built with a new house | § 66311.5(b) |
| Egress expansion on a converted structure | The 150 sq ft counts toward the 750 | ADU Handbook, Fees |
What an "impact fee" actually is
Impact fees are the charges a city levies to help pay for public facilities serving new development — parks, roads, libraries, fire stations. They are determined under the Mitigation Fee Act, and the term covers any monetary exaction other than a tax or special assessment charged in connection with approving the ADU for the purpose of defraying the cost of public facilities relating to it (ADU Handbook, Fees). Park dedication charges — Quimby fees — are folded into the definition for this purpose.
What impact fees are not: your building permit fee, plan check fee, or the cost of inspections. Those are processing charges, and they are payable regardless of unit size. The exemption below removes a category of fee, not the whole counter bill.
The exemption, and the word "interior livable"
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 ADU with 500 square feet of interior livable space or less (§ 66311.5(c)(1)).
Two features of that sentence are worth slowing down on.
"Or less," not "less than." A unit of exactly 750 square feet is exempt, and the measure is interior livable space rather than gross floor area. This wording is recent, and you will find both versions in circulation: the state's own handbook page on fees is still phrased around an ADU "less than 750 square feet" (ADU Handbook, Fees), while the current statute reads "750 square feet of interior livable space or less" (§ 66311.5(c)(1). The statute is what controls — but if your city is working from older guidance, expect to have to point at it.
The 150-square-foot expansion counts. An ADU created inside an existing accessory structure may be expanded up to 150 square feet beyond the structure's dimensions solely for ingress and egress. Those 150 square feet count toward the 750. The state's own worked example: a 700-square-foot interior conversion plus a 150-square-foot egress addition is treated as an 850-square-foot ADU for fee purposes, which triggers the proportionate fee requirement (ADU Handbook, Fees). That is the accidental way over the line.
Above the line: proportional, not full
Crossing 750 square feet does not mean paying what a new house pays. Any impact fee charged on a larger ADU must be charged proportionately in relation to the square footage of the primary dwelling unit (§ 66311.5(c)(1)).
The state illustrates it plainly: a 2,000-square-foot primary dwelling with a proposed 1,000-square-foot ADU could result in 50 percent of the impact fee that would be charged for a new primary dwelling on the same site (ADU Handbook, Fees). If your city quotes a full new-dwelling impact fee for an ADU of any size, that is a question worth asking in writing.
Utility connections are a separate track
Connection fees and capacity charges — the water and sewer district's charges for hooking up — are not impact fees, and they follow their own rule. An ADU or JADU shall not be considered a new residential use for the purpose of calculating them, unless the unit was constructed with a new single-family dwelling (§ 66311.5(b). Where a charge does apply, it must be proportionate to the burden of the proposed ADU, based on its square footage or plumbing fixtures compared with the primary dwelling (ADU Handbook, Fees).
Conversions get a stronger protection. For an ADU or JADU created within existing space under Government Code section 66323(a)(1), a local agency, special district or water corporation may not require a new or separate utility connection directly between the unit and the utility, or impose a related connection fee or capacity charge — unless the unit was built with a new single-family dwelling, or upon separate conveyance of the ADU (§ 66311.5(d)).
School fees have their own, lower line
School districts may levy fees on ADUs, and their cutoff sits at 500 square feet, not 750. An ADU or JADU containing less than 500 square feet of interior livable space is treated as residential construction that does not increase assessable space by 500 square feet, which is what keeps it outside the school fee calculation (§ 66311.5(c)(3)). Districts are authorized — but not required — to charge for units above that size, and the state adds that local agencies should not withhold an ADU or JADU permit because of school fees (ADU Handbook, Fees).
So a unit between 500 and 750 square feet is exempt from local impact fees but can still see a school fee. It is the one gap in the middle of the range.
How the size interacts with what you're allowed to build
The fee cutoff is not a size limit — it is a price signal. Separately, a city's maximum unit size must allow an ADU of at least 850 square feet, or 1,000 square feet for an ADU with more than one bedroom, and lot coverage or floor area ratio rules cannot preclude an ADU of at least 800 square feet maintaining four-foot rear and side setbacks (ADU Handbook, Size Requirements). Minimum lot size requirements may not be imposed on ADUs at all.
In other words, you are entitled to build bigger than 750 square feet in most cases. The decision is whether the extra room is worth the fee bracket — which is exactly the calculation the Legislature wanted homeowners to be able to make. If your city's fee schedule appears not to follow these rules, the California Department of Housing and Community Development reviews local ADU ordinances for compliance; hcd.ca.gov is where to raise it.
