San Marcos, CA Accessory Structures: ADU Rules (2026)
ADU rules in San Marcos, CA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.
Key Facts
- One-bedroom ADU cap
- 850 square feet
- Multi-bedroom ADU cap
- 1,000 square feet
- Junior ADU cap
- 500 square feet
- Detached ADU height
- 16 ft, up to 20 ft near transit
- Minimum side/rear setback
- 4 feet for new detached ADU
Summary
San Marcos caps accessory dwelling units under Municipal Code § 20.410.060 at 850 square feet for a one-bedroom unit and 1,000 square feet for two or more bedrooms, or 50% of the primary home's living area, whichever is smaller. Detached ADUs top out at 16 to 20 feet in height depending on transit proximity.
C. Size and Height Limitations. The building size for all ADUs shall be limited to the smallest square footage applicable to the parcel:1.Size of attached or detached ADU shall be:a.The total floor area of an ADU shall not exceed fifty percent (50%) of the living area of the proposed or existing primary dwelling, provided, however, that the maximum square footage allowed must be at least eight hundred fifty (850) square feet of gross floor area.b.The total square footage for an ADU providing one bedroom shall not exceed eight hundred fifty (850) square feet of gross floor area.c.The total square footage for an ADU that provides more than one bedroom shall not exceed one thousand (1,000) square feet of gross floor area.
Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: Code of Ordinances: Supplement 14 Update 1).
Full Breakdown
410. a caps an attached or detached ADU's total floor area at 50 percent of the primary dwelling's living area, but guarantees at least 850 square feet regardless of how small that percentage works out to. c). a). b). a). E). A).
Violations & Fines
Building an ADU that exceeds the applicable square-footage or height cap, or that doesn't meet the reduced 4-foot setback and 10-foot separation standards, is a zoning code violation that can block issuance of a building permit or certificate of occupancy. An ADU built without permits or in violation of these standards is subject to the same enforcement remedies as any unpermitted structure, including stop-work orders and required correction before the unit can be legally occupied.
Frequently Asked Questions
How big can an ADU be in San Marcos?
How tall can a detached ADU be?
What setbacks apply to a new detached ADU?
Sources & Official References
Other rules in San Marcos
How San Marcos compares: Easiest Cities to Build an ADU·California rules heatmap·Compare San Marcos to another location·View the California accessory structures overview
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ADU Rules in Nearby Cities
How other cities in San Diego County handle adu rules.