ADU Impact Fees in California (and the Under-750 sq ft Exemption) | 1-800-ADU-Pros
ADU impact fees in California: units under 750 sq ft are exempt under SB 13. See which permit fees still apply, the school-fee surprise, and how to keep costs down.
Written by 1-800-ADU-Pros
ADU Impact Fees in California (and the Under-750 sq ft Exemption)
Under 750 sq ft, you pay no impact fees at all — that's state law (SB 13). Here's every fee that still applies to a California ADU, and how to keep them down.
We match you with vetted, California-licensed ADU builders in your area — no guesswork, no cold-calling contractors.
"Impact fees" is one of the scariest line items people imagine when they price out an ADU — partly because for a brand-new house, those fees can run into the tens of thousands. The good news for California homeowners: the state deliberately gutted impact fees for small ADUs to get more of them built. If your unit is small enough, the impact-fee line is simply zero.
This guide demystifies the jargon. We'll separate true impact fees from the permit and plan-check fees the building department charges, and from the utility connection fees for water and sewer — three different buckets that get lumped together and confuse everyone. Then we'll show you which fees still apply, the one fee that surprises people (school fees), and how to legally minimize the whole stack.
The 30-second answer
In California, ADUs under 750 sq ft are exempt from impact fees by state law (SB 13, effective 2020). ADUs 750 sq ft or larger can still be charged impact fees, but only proportionally to the size of the primary dwelling — not a full new-home fee. Separately, permit/plan-check fees and water/sewer connection fees can still apply at any size. Source: California HCD ADU Handbook.
On this page
- What impact fees actually are
- The under-750 sq ft exemption
- Which fees apply to an ADU
- The school-fee surprise
- How to legally minimize fees
- FAQ
What impact fees actually are
Impact fees (sometimes called development impact fees or capital facilities fees) are one-time charges a local agency levies on new development to help pay for the public infrastructure that new unit is assumed to "impact." Think water and sewer capacity, parks, traffic and transportation, and general public facilities. A brand-new single-family home triggers the full menu; that's why new construction carries such a heavy fee load.
The trap is that homeowners hear "fees" and picture all of these as one number. In reality there are three separate buckets, and they behave very differently for an ADU:
- Impact fees — the capacity/infrastructure charges above. These are the ones SB 13 waives under 750 sq ft.
- Permit & plan-check fees — what the building department charges to review your plans and inspect the work. These are not impact fees and still apply at any size.
- Utility connection fees — the cost to physically hook the ADU up to water and sewer (a new lateral, a meter, sometimes a pump). These can still apply even on a sub-750 sq ft unit.
That last bucket is the one people under-budget the most. Site work and utilities — trenching, a separate meter, a sewer lateral — commonly run about 10–15% of total project cost and are the single most under-estimated item on an ADU job (Snap ADU on sitework cost drivers). We break those numbers down in our ADU utility & sewer hookup cost guide. The key point: dodging impact fees does not mean dodging the hookup bill.
The under-750 sq ft exemption (the biggest fee break)
This is the single most valuable thing to know about ADU fees in California. Under SB 13 (in effect since January 2020), any ADU under 750 sq ft cannot be charged impact fees at all — not for parks, not for traffic, not for capital facilities. The state did this on purpose to make small ADUs cheaper and faster to build.
For ADUs 750 sq ft and larger, impact fees are still allowed, but the law caps how they're calculated: the fee must be charged proportionally to the square footage of the primary dwelling. So a 900 sq ft ADU behind a 2,000 sq ft house is assessed at a fraction of the home's fee — never as if it were a whole new house dropped on the lot.
The 749-vs-751 cliff is real money
Because the exemption is a hard line at 750 sq ft, the difference between a 749 sq ft ADU and a 751 sq ft ADU isn't 2 square feet of floor — it's potentially thousands of dollars in impact fees that switch on the moment you cross the threshold. If your design is hovering near 750, it's worth a serious conversation about whether the extra space is worth the fees it unlocks.
State law guarantees you can build an ADU of at least 800 sq ft regardless of local limits, and the City of LA allows detached ADUs up to 1,200 sq ft — see our ADU size limits in Los Angeles guide for the full picture. The takeaway: bigger is allowed, but it isn't free. The fee math is a real input into how big you should go.
Which fees apply to an ADU
Here's the honest breakdown of each fee type, whether it applies to your ADU, and the catch to watch for. Always confirm the exact numbers with your city and school district — these vary by jurisdiction and by year.
| Fee type | Applies to ADU? | Notes |
|---|---|---|
| Impact fees (traffic, park, capital facilities) | Only if ≥ 750 sq ft | Waived entirely under 750 sq ft (SB 13). At 750+, charged proportionally to the primary home's size, not a full new-home fee. |
| School district fees | Often, if ≥ ~500 sq ft | Levied by square footage; many districts charge them on ADUs at/above ~500 sq ft. The fee people forget. Verify with your district. |
| Building permit & plan check | Yes | Not an impact fee — this is the building department's review/inspection charge. Applies at every size. |
| Utility connection (water/sewer) | Often | Hookup, meter, sewer lateral. Can apply even under 750 sq ft. Most under-budgeted item (~10–15% of project). |
| Other (addressing, etc.) | Sometimes | Small administrative charges — new address assignment, recording. Minor, but they exist. |
For how these fold into the full project budget, see our complete ADU cost breakdown for Los Angeles.
The school-fee surprise
Here's the one that catches people off guard. Even when SB 13 wipes out your impact fees, school facilities fees are a separate program — and many California school districts do charge them on ADUs at or above roughly 500 sq ft. They're assessed by the square foot, so a larger unit means a larger bill.
Budget for school fees — then verify the rate
Residential school fees are commonly cited around ~$4–$5 per square foot, but the exact rate varies by district and changes from year to year — treat that as a planning estimate only, and confirm the current number directly with your local school district before you finalize your design. On a 600 sq ft ADU, that's a meaningful line item people routinely forget to plan for.
Why does this trip people up? Because the headline "ADUs under 750 sq ft are exempt from impact fees" is true — but school fees aren't impact fees, so the exemption doesn't touch them. Knowing this upfront is the difference between a budget that holds and a surprise invoice at permit pickup. A vetted local builder who works your city every week will know your district's current rate; that's exactly the kind of local detail our LA ADU permit guide walks through.
Not sure which fees your property will trigger?
Fees depend on your size, your city, and your school district. We'll check your address for free and connect you with a vetted, California-licensed LA builder who knows your jurisdiction's exact numbers.
How to legally minimize your ADU fees
None of this is about gaming the system — the state built these breaks into the law on purpose to encourage ADUs. Here's how to take full advantage:
- Design under 750 sq ft. This is the big one. Staying under the threshold zeroes out impact fees entirely. If you don't need the extra space, the fee savings often make a smaller unit the smarter build.
- Consider a JADU. A junior ADU is up to 500 sq ft carved out of the existing house. Because it lives inside your current footprint, it avoids a separate utility connection and many of the fees a detached unit triggers. See ADU vs JADU to weigh the trade-offs.
- Convert an existing garage. A conversion reuses an existing structure, which often means fewer new-capacity impacts than ground-up construction — and frequently a simpler utility tie-in.
- Confirm your specific city and district before you lock in size. Fee schedules are local. The jump from 749 to 751 sq ft, or from 499 to 501, can switch on real money. Get the actual numbers before you finalize the footprint, not after.
One 2026 process note: SB 543
Effective January 1, 2026, SB 543 requires your city to make a completeness determination within 15 business days of your ADU application. That's a speed/process improvement — it tells you faster whether your submittal is complete — not a fee change, and it does not mean a 60-day finished permit for a custom build. Plan your timeline realistically.
The bottom line: California has deliberately stripped fees to get ADUs built, and SB 13's impact-fee waiver under 750 sq ft is the single biggest break you'll get. Stack it with smart design and a builder who knows your jurisdiction, and the fee line on your ADU can be a lot smaller than the internet scared you into expecting. Run your address through our free qualification check and we'll point you to a vetted local pro who can give you real, jurisdiction-specific numbers.
ADU impact fee questions
Are ADUs exempt from impact fees in California?
Yes, if the ADU is under 750 sq ft. Under SB 13 (effective 2020), California prohibits local agencies from charging impact fees on any ADU smaller than 750 sq ft. ADUs of 750 sq ft or larger can still be charged impact fees, but only proportionally to the size of the primary dwelling — never as a full new-home fee.
What's the difference between impact fees and permit fees?
Impact fees are one-time charges that fund public infrastructure (parks, traffic, water/sewer capacity, capital facilities) the new unit is assumed to impact. Permit and plan-check fees are what the building department charges to review your plans and inspect the work. They are separate buckets: SB 13 waives impact fees under 750 sq ft, but permit and plan-check fees still apply at any size.
Do I still pay school fees on a small ADU?
Often, yes. School facilities fees are a separate program from impact fees, so the SB 13 under-750 exemption does not cover them. Many California school districts charge them on ADUs at or above roughly 500 sq ft, assessed by the square foot. The rate is commonly cited around $4 to $5 per square foot, but it varies by district and year — verify the current number with your local district.
Do utility connection fees still apply under 750 sq ft?
They can. Water and sewer connection fees pay to physically hook the ADU up — a new lateral, a meter, sometimes a pump — and are separate from impact fees, so they can apply even on a sub-750 sq ft unit. Site work and utilities are commonly about 10 to 15 percent of total project cost and are the most under-budgeted part of an ADU build.
Does SB 543 reduce ADU fees in 2026?
No. SB 543, effective January 1, 2026, requires your city to determine whether your ADU application is complete within 15 business days. That is a process and speed improvement, not a fee change. It does not waive any fees, and it does not guarantee a finished permit in 60 days for a custom build.
