San Mateo County, CA Accessory Structures: ADU Rental Restrictions (2026)
Key Facts
- Governing section
- Zoning Regulations § 8.392.050(22)
- Minimum rental term
- Longer than 30 days
- Applies to
- All ADUs approved under Ch. 8.392
- Short-term stays
- Not permitted in an ADU
- Scope
- Unincorporated San Mateo County only
Summary
San Mateo County bars accessory dwelling units built under its ADU chapter from short-term rental: Zoning Regulations Section 8.392.050(22) requires any ADU that is rented to be rented for a term longer than thirty (30) days, ruling out nightly or weekly stays regardless of the unit's eligibility for a short-term rental permit elsewhere in the code.
22. Short Term Rental. Accessory dwelling units created pursuant to the provisions of this Chapter, if rented, shall only be rented for a term longer than thirty (30) days.
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
392, draws a hard line between ADUs and the county's separate short-term rental program: accessory dwelling units created under this chapter, if rented, shall only be rented for a term longer than thirty days. 368, defines a short-term rental, a unit rented for fewer than thirty consecutive days. 050 ADU. The restriction runs with the unit for as long as it exists as an ADU under this chapter; it is not tied to owner-occupancy of the primary residence, and it applies whether the ADU is attached, detached, or a converted garage.
The rule sits alongside the chapter's other permanence conditions, including the parking, setback, and floor-area terms that make an ADU eligible for ministerial approval in the first place, and it governs only unincorporated San Mateo County; incorporated cities set their own ADU rental-term rules under their own municipal codes.
Violations & Fines
Renting an ADU for stays under thirty-one days breaches the Section 8.392.050(22) condition attached to the unit's ministerial approval and functions as an unpermitted short-term rental: the county can pursue the same code-enforcement and civil remedies used against any zoning violation, and advertising the ADU for short-term stays without the separate Chapter 8.368 short-term rental permit exposes the owner to that chapter's own penalties.
Frequently Asked Questions
Can I list my backyard ADU on a short-term rental platform?
Why does San Mateo County bar short-term ADU rentals?
Does the 30-day rule apply if I live in Redwood City or San Mateo?
What happens if I rent my ADU short-term anyway?
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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