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ADU impact fees too high? Check the rules first

By the HyreADU research desk. Last checked .

The short answer

If ADU impact fees look too high, check the size first. In California, no impact fee may be charged on an ADU of 750 square feet or less of interior livable space, and above that impact fees must be proportional to the main house (Gov. Code § 66311.5).

Connection and capacity charges and school fees are separate rules, so a bill can include them legally.

Why it happens

  • The ADU is over 750 sq ft, so proportional impact fees apply.
  • The bill includes connection or capacity charges, which are not impact fees.
  • School fees under a separate statute.
  • Plan check and permit processing fees, which are also separate.
  • An error: impact fees charged on a unit at or under 750 sq ft.

What to do this week

  1. Ask for an itemized fee list with the authority for each line.
  2. Check the interior livable area on your plans.
  3. If the unit is 750 sq ft or less, ask why any impact fee applies.
  4. If above, ask how the proportional calculation was done.
  5. Ask the water and sewer agencies how their charges were calculated.

Design changes that lower fees

Reducing the unit to 750 square feet or less of interior livable space removes impact fees in California. Many agency plans sit just under the line. Run your numbers in the ADU fee calculator and the ADU impact fee checker, and see fees by city.

Who fixes it

The city’s fee staff can correct errors. For a dispute over whether the law was applied correctly, seek legal advice; we give none.

A mistake to avoid

The mistake to avoid is paying a fee bill you do not understand. Ask for each line and its authority before paying. Fees paid under protest or later disputes are legal questions; a clear itemized list is the first step either way.

What to do next: check your lot with the ADU feasibility checker, read the questions to ask an ADU builder, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.

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Questions

Are ADUs exempt from impact fees?

In California, ADUs of 750 sq ft or less are. Larger ones pay proportional fees.

Are connection fees impact fees?

No. They are a separate category and can still be charged.

Do JADUs pay impact fees?

Not at 500 sq ft or less.

Do school fees apply?

Under a separate statute; an ADU under 500 sq ft is treated as not adding 500 sq ft of assessable space.

Sources

HyreADU is a publisher. It does not design, permit or build ADUs, has no partner builders, and gives no legal, tax, lending or engineering advice.

Rules vary by state and city; California rules are labeled as such. Spotted an error? Tell us through our corrections page. Licensed professionals can also contribute as named sources.