San Mateo County, CA Accessory Structures: ADU Rules (2026)
ADU rules in San Mateo County, CA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- Governing chapter
- Zoning Regulations Ch. 8.392
- Allowed districts
- R-1, R-2, R-3, PC, CMU-1/2/3, NMU, RE, RH, RM, TPZ
- Approval path
- Ministerial, no hearing, if standards met
- Max height
- 26 feet
- Min side/rear setback
- 4 feet
- Guaranteed minimum size
- 800 sq ft cannot be blocked
Summary
San Mateo County allows accessory dwelling units by right in the R-1, R-2, R-3, PC, CMU, NMU, R-E, RH, RM and TPZ districts and any other residential district under Zoning Regulations Section 8.392.030, and approves ADUs meeting Section 8.392.050's development standards ministerially, with no public hearing, under Section 8.392.040.
Accessory dwelling units shall be allowed in the R-1, R-2, R-3, PC, CMU-1, CMU-2, CMU-3, NMU, NMU-ECR, R-E, RH, RM and TPZ districts, and all other districts in which residential uses are permitted, regardless of any regulations that might otherwise prohibit accessory dwelling units in those districts. Accessory dwelling units meeting all of the applicable requirements of Sections 8.392.050 through 8.392.090, shall be approved ministerially, without public notice, public hearing, or discretionary review. Accessory dwelling units not meeting the applicable standards set forth in Section 8.392.050 through 8.392.090 will be considered a conditionally permitted use within the districts specified in Section 8.392.030 and may be permitted by a conditional use permit pursuant to a public hearing before the Zoning Hearing Officer, as described in Section 8.392.110.
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
030, overriding any district-specific rule that would otherwise bar them. 110. 050 are specific: ADUs are exempt from minimum lot area, minimum lot size, and maximum density limits; minimum side and rear setbacks are 4 feet, with a 5-foot side and 10-foot rear stepback once the structure passes 16 feet in height; no lot-coverage or setback rule may block an 800-square-foot ADU; and maximum height is capped at 26 feet, measured from the lower of finished or natural grade. Every ADU built under this chapter must include independent living, sleeping, eating, cooking, and sanitation facilities, a kitchen with a fixed stovetop, and a fully plumbed bathroom.
This chapter applies only where the county, not a city, holds zoning authority, the unincorporated pockets around Redwood City, San Mateo, and the county's other incorporated cities; each of those cities administers its own ADU ordinance under its own municipal code.
Violations & Fines
Building an ADU that departs from Section 8.392.050's standards without first obtaining a conditional use permit under Section 8.392.110 is an unpermitted structure, exposing the owner to a stop-work order, permit denial, and required conformance or removal before any certificate of occupancy issues. Skipping the ministerial review entirely, even for a compliant design, is still a zoning violation subject to the county's standard code-enforcement remedies.
Frequently Asked Questions
Can I build an ADU on my unincorporated San Mateo County lot?
Do I need a public hearing to get ADU approval?
How tall and how close to the property line can an ADU be?
Does this chapter apply if I live in the City of San Mateo?
Sources & Official References
Other rules in San Mateo County
How San Mateo County compares: Easiest Cities to Build an ADU·California rules heatmap·Compare San Mateo County to another location·View the California accessory structures overview
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