San Mateo County · California
San Mateo County (unincorporated) ADU Fees in 2026
Every government fee we could verify for an ADU in San Mateo County (unincorporated), from official sources. Figures below are for an 800 sq ft ADU unless noted.
Every figure links to its official source document. Last verified 18 Oct 2026. Fees are re-verified on a rolling schedule (next full pass Dec 2026). Official fee schedule ↗ How we price →
Verified fees, 800 sq ft ADU
$12,450
across 6 line items
The 750 sq ft cliff
$0*
no verified impact fees on file yet
Fee table
Amounts computed for an 800 sq ft ADU from each item's published rate. "Exempt" lines are $0 by state law at this size.
| Fee | Category | Rate | 800 sq ft | Source |
|---|---|---|---|---|
| Menlo Park City SD + Sequoia Union HSD combined Level I fee (residential) | School fees | $5.38/sq ft | $4,304 | official doc ↗verified 23 Oct 2026 |
| Affordable Housing Impact Fee - second unit (ADU) | Impact fees | flat fee | $0 | official doc ↗verified 18 Oct 2026 |
| Building permit (residential buildings sq-ft schedule) | permit | $8.33/sq ft | $6,664 | official doc ↗verified 18 Oct 2026 |
| Application filing fee - full review (new residential structures) | permit | flat fee | $595 | official doc ↗verified 18 Oct 2026 |
| Planning review of building permit plans | Planning fees | flat fee | $887 | official doc ↗verified 18 Oct 2026 |
| Design review - state-exempt ADU | Planning fees | flat fee | $0 | official doc ↗verified 18 Oct 2026 |
These are the line items we have verified so far — not a complete bill. Site-specific charges (utility laterals, soil work) vary by lot. How we price →
What changes on January 1, 2027
Two state laws rewrite parts of this table. SB 1117 (Chapter 809) re-bases covered impact fees for up to two ADUs on only the square footage above 750 — at 800 sq ft, most impact-fee lines shrink to a 50 sq ft basis. School developer fees and water/sewer connection or capacity charges are not covered and do not change. AB 956 (Chapter 791) makes two detached ADUs per single-family lot a ministerial approval. Details: SB 1117 and the 750 sq ft cliff · AB 956: two ADUs on one lot
Common questions
Are impact fees waived for ADUs under 750 sq ft in San Mateo County (unincorporated)?
Yes. California law (Gov. Code §66311.5) bars local agencies from charging impact fees on ADUs of 750 square feet or less. That applies in San Mateo County (unincorporated) the same as everywhere else in California.
Where do these San Mateo County (unincorporated) ADU fee numbers come from?
Every line item links to the official source document — the city or county fee schedule, the school district's developer-fee resolution, or the utility district's charges — with the date we last verified it. We publish no fee modeled, no fee guessed.
How much are school district fees for an ADU in San Mateo County (unincorporated)?
$4,304 for an 800 sq ft ADU ($5.38/sq ft), per the district document linked in the table. Note: since January 1, 2026, ADUs under 500 sq ft are exempt from school fees under state law.
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