How this calculator works
There is barely any arithmetic, and that is the point — the hard part of this question is which rule applies, not the multiplication.
counted = ADU interior livable sq ft + exterior expansion sq ft
cliff = 750 sq ft (ADU) | 500 sq ft (JADU)
IF counted <= cliff
impact fee ceiling = ZERO (§ 66311.5 — prohibited outright)
ELSE
share = counted / primary dwelling sq ft
ceiling = share x (your city’s new-SFD impact fee)
school: separate statute (Ed Code § 17620), threshold 500 sq ft
connection: NOT an impact fee — outside the prohibition entirely;
the "not a new residential use" protection does NOT apply
where built concurrently with a new single-family dwellingWorked, using HCD’s own illustration. A 2,000 sq ft primary dwelling with a proposed 1,000 sq ft ADU: share = 1,000 / 2,000 = 50%, so the impact fee ceiling is half of whatever a new primary dwelling would be charged on that site.
Enter a $25,000 single-family impact fee and the ceiling is $12,500.
Now the cliff. Take that same house with a 760 sq ft ADU instead. The ceiling is 760 / 2,000 = 38%, or $9,500.
Bring the counted area down to 750 and the ceiling is zero — not $9,375, not a proportional trim, but nothing, because the threshold is a cliff and not a taper. That is roughly $9,500 riding on ten square feet of floor.
Every dollar in those examples is illustrative arithmetic on a figure a reader would supply. We publish no city’s actual fee.
What each input means
| Input | What it is actually asking |
|---|---|
| What you are building | Detached, attached, conversion or JADU. It changes two things: a JADU has a 500 sq ft threshold rather than 750, and a unit created from existing space gets the connection-fee protection that a new-build does not automatically get. Your planning department will tell you which category your project falls into if the drawings are borderline. |
| ADU interior livable space | The floor area your planning department counts as interior livable space. WHERE TO FIND IT: your own drawings, but confirm the counting method with the planner before the design is final — this single number decides everything else on this page, and departments differ on what they include. |
| Exterior expansion | Any added exterior area such as a porch, entry or stair enclosure. Enter it separately because it counts toward the threshold: HCD’s worked example is a 700 sq ft conversion plus a 150 sq ft addition counting as 850. Leave at 0 if the whole project is within the existing footprint. |
| Primary dwelling size | The square footage of the main house. Only used when you are above the threshold, where the statute caps the fee at the ADU’s share of the primary dwelling. From your county assessor record or your own plans. |
| Your city’s new single-family impact fee | The total impact fee your jurisdiction charges for a new single-family dwelling. WHERE TO FIND IT: search your city or county name plus "impact fee schedule" or "development impact fees" — the Mitigation Fee Act requires these to be adopted and published, so a document almost always exists. Total the categories (traffic, parks, public facilities, drainage), or ask the permit counter for the total in writing. We hold no municipal fee data. |
| Built concurrently with a new house | Whether the ADU is going up at the same time as a brand-new primary dwelling, rather than beside one that already exists. This is an express statutory carve-out for connection and capacity charges, and answering it wrong can change the utility answer by a large amount. |
Worked examples
Including one where the naive answer misleads, which is the example most calculators leave out.
A 700 sq ft garage conversion — the clean case
Interior conversion, 700 sq ft, no exterior expansion. The tool returns ZERO impact fees may be imposed, because at 700 counted square feet the unit is under the threshold and § 66311.5 prohibits any impact fee from a local agency, special district or water corporation — including Quimby park fees.
The cliff line notes 50 sq ft of headroom and warns to guard it.
But the school line still flags that Education Code § 17620 fees may apply above 500 sq ft, and the connection line still warns that utility capacity charges sit outside the prohibition. Zero impact fees is not zero fees.
The same conversion with a 150 sq ft entry — the trap
Identical 700 sq ft interior conversion, but now with a 150 sq ft exterior addition for ingress. Counted area becomes 850.
The answer flips: impact fees are now permitted, capped at 850 / 2,000 = 43% of the single-family figure — about $10,625 on a $25,000 schedule. Nothing about the living space changed.
This is HCD’s own example, and it is the most expensive 150 square feet in the topic.
Ten square feet over — pricing the cliff
A 760 sq ft detached ADU beside a 2,000 sq ft house. Ceiling: $9,500.
The cliff output says it plainly — bringing the counted area down to 750 takes the ceiling to zero, because the threshold is a cliff and not a taper, so roughly $9,500 rides on ten square feet of floor.
Whether that trade is worth making is the reader’s call; the tool’s job is to make sure it is a decision rather than an accident.
A 450 sq ft JADU — two thresholds at once
A junior ADU of 450 sq ft. Impact fees: prohibited, against the 500 sq ft JADU threshold.
And because 450 is also under the 500 sq ft school threshold, the school line reports that developer fees should not apply either — § 66311.5(c)(3) treats a unit under 500 sq ft as not increasing assessable space.
This is the one configuration where two different statutes happen to point the same way, which is worth knowing precisely because they usually do not.
What changes the result
What moves the answer, ranked.
1. Counted square footage against the threshold. Not a factor among factors — a switch. Everything above it is proportional arithmetic; everything at or below it is zero. No other input can override it.
2. Whether an exterior expansion is counted. The commonest way a compliant design crosses the line, and invisible unless someone tells you the rule.
3. Your city’s single-family impact fee. Pure scale on everything above the threshold, and the input with the widest real-world variation — which is exactly why we make you fetch it rather than assuming it.
4. The ADU-to-primary square footage ratio. Sets the proportion above the threshold. A large ADU beside a small house is proportionally expensive; the same ADU beside a large house is not.
5. Concurrent construction with a new house. Binary, and it governs the connection-fee answer rather than the impact-fee answer.
What is deliberately absent.
No city fee data, of any kind. Not a figure, not a range, not a national average. The verification failure is recorded rather than papered over, and a plausible wrong magnitude would be more damaging than an honest blank, because it is precise enough to be budgeted against.
No permit or plan-check fees. Those are cost-of-service charges under a different logic from impact fees, they vary by jurisdiction, and we hold none.
No prediction of whether your city will charge the ceiling. The statute sets a maximum. What is actually levied is a local policy choice, and plenty of jurisdictions charge less than they could.
No other state. These are California statutes and the tool says so in its own output rather than letting a reader elsewhere assume.
Local considerations
The rule is statewide; the magnitude is entirely local. That split is the reason this page is shaped the way it is. Section 66311.5 binds every California local agency identically — the 750 square foot prohibition applies in every city in the state.
What varies, by an order of magnitude, is the single-family impact fee that the proportional rule works from. One is a fact we can verify and publish; the other is a fact your city publishes and we will not guess.
Special districts and water corporations are covered too. The prohibition is not limited to your city. It expressly reaches special districts and water corporations, which matters because those are frequently the bodies levying the largest charges — and are also, often, a different organization from the one issuing your permit.
But connection and capacity charges are the local wildcard. They sit outside the impact-fee rules entirely, they are frequently levied by a district rather than the city, and in many places they are the largest single line on the permit.
Ask both utilities separately and early, in writing. This is the number most likely to change whether a project pencils, and it is the one we can tell you least about.
When not to use this
Do not use it outside California. Every rule here is a California statute. Nothing on this page describes the law in any other state.
Do not treat the ceiling as a bill. It is the statutory maximum given your inputs. Your jurisdiction may charge less or nothing.
Do not use it as legal advice. It is a screening aid for a conversation at the permit counter. A fee dispute of any size deserves a professional opinion, and this page is not one.
Do not use it to budget a project. It covers impact fees, and flags school and utility charges without pricing them. Permit fees, plan check, utility connection and school fees are all separate lines we do not hold.
Do not assume the counted area is what your plans say. Confirm with the planner what they will count, before the design is final. That single confirmation is worth more than everything else on this page.
HyreADU does not design, permit or build accessory dwelling units. It does not appraise, lend, invest, let property or prepare taxes, it does not rank or refer contractors, and it takes no referral fee from anyone who does any of those things.
Related on this site
- ADU cost calculator Where the fee line sits inside the whole build budget.
- ADU size calculator What you may build, before you choose where to sit against the threshold.
- Bid comparison Whether a bid includes fees or excludes them is the commonest asymmetry between two quotes.
Questions this calculator answers
- Do I have to pay impact fees on an ADU in California?
- Not if the unit is 750 square feet of interior livable space or less — or 500 or less for a junior ADU. Government Code § 66311.5 prohibits a local agency, special district or water corporation from imposing any impact fee on such a unit, and the prohibition expressly includes Quimby park fees. Above that size, impact fees are permitted but must be charged proportionately to the square footage of the primary dwelling.
- What is the 750 square foot ADU rule?
- It is a threshold, not a sliding scale. At or under 750 square feet of counted area, no impact fee may be imposed at all. Above it, the fee must be proportional: a 1,000 sq ft ADU alongside a 2,000 sq ft primary dwelling could be charged about 50 percent of what a new primary dwelling would pay on the same site. Because it is a cliff rather than a taper, one square foot either side of the line can be worth a five-figure difference.
- Does a porch or exterior addition count toward the 750 square feet?
- Yes, and this catches people. California HCD gives the worked example directly: a 700 square foot interior conversion with a 150 square foot exterior expansion for ingress and egress counts as an 850 square foot ADU for fee purposes, which triggers the proportional requirement. Confirm with your planning department exactly what area they will count before the drawings are final.
- Why does my city still cite Government Code 66324?
- Because the section was renumbered. The ADU fee rules sat at former § 65852.2, then at § 66324, and were moved to § 66311.5 by SB 543 (Stats. 2025, ch. 520, effective 1 January 2026). The substance is largely unchanged, so a page citing § 66324 is not necessarily wrong about the rule — but the citation is a useful signal of how recently that page was reviewed. Even the California HCD ADU Handbook we read on 5 September 2026 still cites the old number, because it predates the change.
- Are school fees covered by the ADU impact fee exemption?
- No — they are a different statute with a different threshold. School developer fees are levied under Education Code § 17620, and § 66311.5(c)(3) treats an ADU or JADU under 500 square feet of interior livable space as not increasing assessable space by 500 square feet. So the school threshold is 500 square feet while the impact fee threshold is 750, and a unit between the two can be exempt from one and liable for the other. School districts are authorized to levy these fees but are not required to; ask yours whether it does.
- Can I still be charged water and sewer connection fees?
- Yes. Connection and capacity charges are not impact fees and sit outside the prohibition entirely. Under § 66311.5(b) an ADU is not treated as a new residential use for connection-fee purposes unless it is constructed together with a new single-family dwelling — but the utility can still bill a connection or capacity charge, and where it does, the charge should be proportionate to the burden of the ADU measured by square footage or plumbing fixtures against the primary dwelling. In many places this is the single largest line on the permit, so get both utilities in writing early.
- What if I build the ADU at the same time as a new house?
- That is the express carve-out and it matters a great deal. The protection that stops an ADU being treated as a new residential use for connection fees does not apply where the unit is constructed concurrently with a new single-family dwelling. If that describes your project, expect the utility to treat the ADU as a new connection and price it accordingly.
- How do I find my city’s impact fee figure?
- Search your jurisdiction’s name plus "impact fee schedule" or "development impact fees". California agencies must adopt and publish these under the Mitigation Fee Act rather than setting them at the counter, so a published document almost always exists. You want the total for a new single-family dwelling, because that is the base the proportional rule works from. If the schedule is hard to total, ask the permit counter for that total in writing.
- Does this tool know what my city charges?
- No, and it deliberately publishes nothing on the subject. We were unable to verify any municipal or utility ADU fee schedule, and that finding is recorded openly on this site. Rather than print a "typical" figure that would be wrong in most jurisdictions and precise enough to be planned against, this tool applies a statutory rule we did verify to a number you supply from your own city. Every dollar amount you see is one you typed.
- Is a ceiling the same as a bill?
- No, and the distinction is worth holding on to. The proportional figure this tool produces is the maximum the statute permits, given the numbers you entered. Your jurisdiction may charge less, may charge nothing, may not levy some categories at all. What the ceiling gives you is the number to check an itemized fee quote against — which is why the tool tells you to ask for the square footage proportion used.
- Does any of this apply outside California?
- No. Every rule here is a California statute binding California local agencies. ADU fee treatment in other states is a different question with different answers, and we have not built it. Nothing on this page should be read as describing the law anywhere else.
- What should I do if I think I have been overcharged?
- Ask for the fee quote itemized, with the square footage proportion used to calculate it, in writing. Then compare it against the rule: an impact fee on a unit at or under the threshold, or a proportion calculated on something other than the ADU-to-primary square footage ratio, is the discrepancy worth raising — politely, in writing, with the section number. This is a screening aid for that conversation, not legal advice, and a fee dispute of any size is worth a professional opinion.
Sources and methodology
Figures dated 5 September 2026. Last reviewed .
- California Government Code § 66311.5 (ADU fees) (California Legislature, as held in HyreADU’s verified fact base, retrieved 2026-09-05. The impact fee prohibition at or below 750 sq ft (500 for a JADU), the proportional rule above it, the Quimby inclusion, the connection-fee treatment at subdivisions (b) and (e), and the school assessable-space treatment at (c)(3). Renumbered from former § 66324 by SB 543 (Stats. 2025, ch. 520), effective 1 January 2026. RETRIEVAL NOTE: leginfo renders section text client-side and returned no readable statutory text to automated retrieval on this date; the rules above are carried from this site’s existing verified fact base rather than re-read from leginfo.)
- ADU Handbook Update (California Department of Housing and Community Development, retrieved 2026-09-05. Source of the worked example that a 700 sq ft interior conversion with a 150 sq ft exterior expansion counts as 850 sq ft for fee purposes, and of the proportionality illustration. RETRIEVAL NOTE: hcd.ca.gov returned HTTP 403, consistent with this site’s recorded finding; the document was read at the Assembly Housing and Community Development committee mirror. It predates SB 543 and therefore cites § 66324 throughout, where this page cites § 66311.5.)
- California Education Code § 17620 (school facilities fees) (California Legislature, retrieved 2026-09-05. The separate authority under which school districts may levy developer fees. Cited to establish that school fees are a different statute from the impact fee rules, with a different threshold — not to state a fee amount.)
- Mitigation Fee Act, California Government Code § 66000 et seq. (California Legislature, retrieved 2026-09-05. The framework defining what an impact fee is and requiring fees to be adopted and published — which is why your jurisdiction has a schedule you can look up.)
Related
- ADU cost calculator Fees are one line in a build budget. This one prices the line; that one builds the budget.
- ADU size calculator What you are allowed to build, before you decide where to land against the fee threshold.
- ADU ROI calculator Whether crossing the 750 threshold pays for itself in rent is an ROI question.
- Bid comparison Check whether a bid includes permit fees or excludes them — the commonest bid asymmetry.
- Pre-approved ADU plan finder Pre-approved plans are often drawn to sit under the threshold deliberately.