San Mateo County, CA Accessory Structures: Garage Conversions (2026)
Garage conversion rules in San Mateo County, CA, sometimes called garage-to-ADU or accessory living unit conversions, govern permits, ceiling height, egress, and parking replacement.
Key Facts
- Parking replacement
- Not required after garage conversion
- Use permit
- Not required if standards met
- Governing sections
- § 8.392.050(14)(d) and (20)
- Approval track
- Ministerial, no public hearing
- Scope
- Unincorporated San Mateo County only
Summary
Converting an attached or detached garage into an accessory dwelling unit in unincorporated San Mateo County does not require replacing the lost parking spot: Zoning Regulations Section 8.392.050(14)(d) waives replacement parking entirely, and Section 8.392.050(20) confirms garage-based ADUs need no separate use permit if they meet the chapter's standards.
d. Garage Conversion. If an existing attached or detached garage is converted to an accessory dwelling unit, the parking previously provided by that garage is not required to be replaced, and if voluntarily replaced, may be replaced by uncovered parking of any type and configuration allowed by Section 8.392.050(14)(f), below, and no additional parking related to the accessory dwelling unit is required. For purposes of this Section, conversion includes partial or full demolition of the garage and partial or full replacement with an accessory dwelling unit. 20. Conversion of Accessory Buildings. An accessory dwelling unit may be constructed within, above, or below an existing, detached accessory building, provided the resulting unit conforms to all applicable provisions of this Chapter. Accessory dwelling units built within, above, or below existing, or new garages are subject to the specific provisions of this Chapter regarding such units, regardless of any regulations to the contrary elsewhere in the Zoning Regulations.
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
San Mateo County treats garage conversions as one of the easiest paths to a legal accessory dwelling unit. 050(14)(f); no additional parking tied to the new ADU is required at all. The same subsection defines conversion broadly enough to cover partial or full demolition of the garage combined with partial or full replacement by the ADU, so a full teardown-and-rebuild on the same footprint still qualifies. 050(20) reinforces the same policy from the building-code side: an ADU built within, above, or below an existing detached accessory building, which includes a garage, needs no separate use permit regardless of what the applicable zoning district would otherwise require, and units built within, above, or below existing or new garages are governed by this chapter's specific ADU provisions even where another part of the Zoning Regulations would say otherwise.
050 development standards, the 4-foot side and rear setbacks, the 26-foot height cap, and the independent kitchen and bathroom requirements, moves through Planning and Building with no public hearing. The parking waiver and use-permit exemption apply only in the unincorporated county; a garage conversion inside Redwood City, San Mateo, or another incorporated city is reviewed under that city's own ADU ordinance.
Violations & Fines
Converting a garage into living space without going through the county's ADU permit process, rather than relying on the Section 8.392.050(14)(d) and (20) exemptions, is an unpermitted structure and a zoning violation: the county can require the owner to obtain retroactive approval, correct nonconforming work, or restore the garage, and can withhold a certificate of occupancy until the conversion meets Chapter 8.392's standards.
Frequently Asked Questions
If I convert my garage to an ADU, do I have to build a new parking space?
Do I need a use permit to convert a detached garage into an ADU?
Can I demolish part of my garage and build a bigger ADU in its place?
Does this parking waiver apply if my garage is in the City of 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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