San Mateo County, CA Accessory Structures: ADU Impact Fees (2026)
Key Facts
- Exemption threshold
- Under 750 sq ft: no impact fees
- Larger ADU fee
- Standard fee x (ADU sq ft / primary sq ft)
- Governing section
- Zoning Regulations § 8.392.050(23)
- Guaranteed ADU size
- Up to 800 sq ft cannot be blocked
- Scope
- Unincorporated San Mateo County only
Summary
San Mateo County waives development impact fees entirely for accessory dwelling units under 750 square feet, and for larger ADUs charges only a prorated share: Zoning Regulations Section 8.392.050(23) sets the fee at the standard impact fee multiplied by the ADU's square footage divided by the primary dwelling's square footage.
23. Impact Fees. Accessory dwelling units of less than seven hundred fifty (750) square feet in size shall be exempt from all impact fees. Accessory dwelling units of greater than seven hundred fifty (750) square feet in size shall only be charged impact fees in an amount equal to the standard impact fee for such a unit, multiplied by the proportion of the square footage of the accessory dwelling unit to the square footage of the primary dwelling unit.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Code of Ordinances: Supplement 47).
Full Breakdown
050(23). Any ADU under 750 square feet is exempt from all impact fees, full stop, whether it is a new detached structure, an addition, or a garage conversion. An ADU larger than 750 square feet is not exempt, but it does not pay the standard flat impact fee either: the fee is calculated as the standard impact fee for a unit that size, multiplied by the proportion the ADU's square footage bears to the square footage of the primary dwelling unit on the same parcel.
In practice that formula ties the fee to how big the ADU is relative to the house it sits behind rather than treating it as a second full dwelling for fee purposes, so a modest ADU behind a large primary residence pays a small fraction of the standard fee even once it crosses the 750-square-foot line. 050(5) guarantees that no floor-area rule can block an ADU of up to 800 square feet, meaning many ADUs sized at or near that guaranteed minimum will still land above the 750-square-foot fee-exempt threshold and owe the prorated fee rather than nothing.
392 in unincorporated San Mateo County; incorporated cities calculate their own ADU impact fees under their own fee schedules and municipal codes, which can differ from the county's 750-square-foot cutoff.
Violations & Fines
There is no fine tied to Section 8.392.050(23) itself: it is a fee formula, not a prohibition. But an owner who understates an ADU's square footage to claim the under-750-square-foot exemption, or who occupies the unit before paying a fee actually owed on a larger ADU, is misrepresenting the permit application and can have the permit conditioned, suspended, or referred for code enforcement until the correct fee is paid.
Frequently Asked Questions
Do I owe impact fees on a small ADU?
What if my ADU is 900 square feet?
Why would my ADU be over 750 square feet if the guaranteed size is 800?
Does the 750-square-foot fee exemption apply inside Redwood City or San Mateo?
Sources & Official References
Other rules in San Mateo County
California rules heatmap·Compare San Mateo County to another location·View the California accessory structures overview
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