Two different fee categories, two different rules
Homeowners often lump "impact fees" together, but California ADU law actually treats general development impact fees (things like traffic, water, and sewer capacity charges) and school district impact fees as two separate categories, each with its own exemption threshold.
General development impact fees
ADUs with 750 square feet or less of interior livable space remain exempt from general development impact fees — this includes development fees, connection fees, and capacity charges imposed by cities, counties, and special districts. This is the exemption most homeowners have already heard about, and it hasn't changed. Above 750 square feet, how a city charges fees varies — Murrieta, for example, publishes a proportional formula (a 1,000 sq ft ADU on a 2,000 sq ft home is billed on roughly a 50% basis); see our Murrieta ADU guide.
School impact fees: what SB 543 changed
| ADU Size | School Impact Fee Treatment (Effective Jan 1, 2026) |
|---|---|
| 500 sq ft or less | Fully exempt from school impact fees |
| 501–750 sq ft | School districts may still charge fees |
| Over 750 sq ft | Fees must be charged proportionally, based on the ADU's square footage relative to the primary dwelling — not at the full new-home rate |
Before SB 543, it wasn't always clear whether space like garages, porches, or storage areas counted toward these size thresholds. SB 543 codifies "interior livable space" as the specific measurement standard, resolving that ambiguity statewide.
Check your ADU's exemption status
Impact-fee exemption checker
Interior livable space is the SB 543 measurement standard — garages, porches and storage areas don't count toward it.
At 750 sq ft or less, cities, counties and special districts can't charge development impact fees, connection fees, or capacity charges.
Between 501 and 750 sq ft, school districts may still charge fees — the amount comes from your district's fee schedule.
Statutory thresholds under California ADU law and SB 543 (effective January 1, 2026). This shows which fee categories can legally apply at your size — actual fee amounts are set locally and this is not a fee quote for your project.
Why the proportional calculation matters
For ADUs larger than 750 square feet, school districts generally can't charge the same flat fee they'd charge for a full new single-family home. Instead, the fee must be calculated proportionally to the relationship between your ADU's square footage and your primary home's square footage — which, in practice, is typically far lower than a standard new-construction school fee. The exact calculation depends on your specific school district's fee schedule, so get the actual number from your district rather than estimating.
This is general information, not a fee calculation for your project
Impact fee schedules are set locally by cities, water districts, and school districts, and can vary significantly between jurisdictions even within the same county. This page describes the general statewide legal framework, not your specific fee amount. Confirm your actual fees with your city's building department and local school district before finalizing your budget.
Frequently asked questions
Not if your ADU has 500 square feet or less of interior livable space — SB 543 exempts these units entirely, effective January 1, 2026. Larger units may still be charged fees, generally on a proportional basis above 750 square feet.
No — they're separate. General development impact fees are exempt up to 750 square feet. School impact fees have their own, lower 500-square-foot full-exemption threshold under SB 543.
Generally no — SB 543 codifies "interior livable space" as the measurement standard, which is intended to exclude garages, porches, and similar non-livable areas from the size calculation.