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San Diego County, CA Accessory Structures: ADU Rules (2026)

Some Restrictions

Compiled from the official code textEditor Martyn O'NeillLast verified September 2026

ADU rules in San Diego County, CA, also called accessory dwelling unit regulations or granny flat ordinances, cover setbacks, owner-occupancy, parking, and permit requirements.

Key Facts

Attached ADU cap: 50% of primary home or 1,200 sf
Detached ADU cap: 1,200 square feet max
Detached ADU height: 25 feet maximum
Setback: 4-foot side/rear setback required
Parking: One space, several exemptions apply

ADU Eligibility Quick-Check

An Accessory Dwelling Unit (ADU) is a smaller, independent living unit on the same lot as a single-family home: converted garages, detached cottages, basement or attic conversions, and attached additions all qualify. Most states have streamlined ADU permitting since 2020, but local rules still control size, height, and setback.

Is your lot zoned single-family or multi-family residential?
Does your lot have at least 1,200 sq ft of buildable space?
Is your existing home owner-occupied (in most states this is not required, but some require it for ADU permits)?
Will the ADU stay under your state's size cap (commonly 850-1,200 sq ft for detached)?
Can you provide one off-street parking space (waived in many states within 0.5 mi of transit)?

If you answered 'yes' to most of these, you likely qualify in California under recent ADU streamlining laws. The San Diego County, CA rules below cover local size, height, and setback requirements.

Summary

Attached ADUs in unincorporated San Diego County are capped at 50% of the primary residence's floor area, up to 1,200 square feet; detached ADUs are capped at 1,200 square feet regardless of the primary home's size.

City-specific rules exist: El Cajon, Vista, San Marcos, Escondido, Oceanside, Carlsbad, San Diego, and Bostonia have their own adu rules rules that differ from San Diego County's county-level regulations. If you live in one of those cities, check the city-specific page instead.

4. The total floor area of an attached ADU shall not exceed 50% of the floor area of the primary residence, up to a maximum floor area of 1,200 square feet. The 50% square foot limitation is not applicable for attached ADUs up to 850 square feet or up to 1,000 square feet if it has more than one bedroom. 5. The total floor area of a detached ADU shall not exceed 1,200 square feet, independent of the square footage of the existing primary residence.

Source: County ADU HandbookView official code

Official source re-checked September 7, 2026: no newer edition of the code had been published (publisher’s edition: 2026 S-141: Administrative: Current through Ord. No. 11007 (N.S.), effective 7-10-26 | Regulatory: Current through Ord. No. 11008 (N.S.), effective 7-24-26).

Full Breakdown

Under Zoning Ordinance § 6156.x, an attached ADU in unincorporated San Diego County can't exceed 50% of the primary residence's floor area or 1,200 square feet, whichever governs, the 50% cap doesn't apply to ADUs up to 850 square feet (1,000 with more than one bedroom). Detached ADUs are capped at 1,200 square feet regardless of the primary home's size and limited to 25 feet in height. Newly built ADUs need a 4-foot side and rear setback plus one on-site parking space, though several exemptions, transit proximity, historic districts, car-share access, waive the parking requirement. Permits run through Planning and Development Services.

Violations & Fines

Building or expanding an ADU without a permit is a zoning violation; Planning and Development Services can require corrective permits, order construction halted, or refer the property for county code enforcement action and fines.

Frequently Asked Questions

Can I build a detached ADU in unincorporated San Diego County without a permit?

No. Zoning Ordinance § 6156.x requires ministerial or discretionary ADU permit approval through Planning and Development Services before construction, even though environmental review is typically waived for qualifying ADUs.

How big can an attached ADU be in unincorporated San Diego County?

Up to 50% of the primary residence's floor area or 1,200 square feet, whichever applies, but ADUs of 850 square feet or less (1,000 with more than one bedroom) are exempt from the 50% cap.

Sources & Official References

[1]San Diego County Zoning Ordinance § 6156.x (amlegal)

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All San Diego County rules

How San Diego County compares: Easiest Cities to Build an ADU·California rules heatmap·Compare San Diego County to another location·View the California accessory structures overview

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ADU Rules in Cities Across San Diego County

San Diego, CA Chula Vista, CA Oceanside, CA Escondido, CA Carlsbad, CA El Cajon, CA Vista, CA San Marcos, CA Encinitas, CA Bostonia, CA