What Is the Maximum ADU Size in San Diego?

Direct Answer

For a new attached or detached ADU in San Diego, the maximum gross floor area is generally 1,200 square feet.

That number is not the whole feasibility answer. Existing structure conversions, the state-protected 800-square-foot ADU pathway, JADU limits, setbacks, height, fire access, and FAR can all affect what is actually buildable on a specific lot.

Attached ADUs

Attached ADUs are connected to the primary dwelling. They may be subject to size rules tied to the main home's floor area, while still being capped at the same general 1,200-square-foot maximum.

In practice, attached ADU feasibility usually depends on:

• the existing home's size and layout
• whether the work is an addition or an interior conversion
• floor area ratio and lot coverage constraints
• egress, fire separation, and utility configuration

Conversions and Existing Structures

Converting an existing permitted garage, accessory structure, or portion of the primary residence can be treated differently from new detached construction.

San Diego guidance states that conversion of an existing accessory structure or a portion of the existing primary residence is not subject to the standard 1,200-square-foot maximum. An ADU built within an existing structure may also add up to 150 square feet when needed only for ingress and egress.

FAR and Lot Coverage

San Diego includes specific FAR exceptions for one ADU of up to 800 square feet. These exceptions can matter on older or already-developed lots where the main dwelling may already be close to, or over, the normal FAR limit.

This issue is especially important for:

• small urban properties
• dense residential neighborhoods
• lots with existing additions
• sites where a 1,200-square-foot ADU is technically allowed but impractical

The 800-Square-Foot Rule

California ADU law and San Diego guidance both include important protections for one ADU of up to 800 square feet. In many cases, a local FAR, lot coverage, open-space, or front setback standard cannot be used to prevent at least an 800-square-foot ADU that otherwise meets the required state-law criteria.

Detached ADUs

Detached ADUs are often the clearest path for backyard rental units, guest housing, multi-generational living, or long-term property flexibility. In San Diego, a detached ADU may be up to 1,200 square feet, subject to the applicable zoning, building, and site constraints.

What About JADUs?

Junior ADUs are different from standard ADUs. A JADU is generally limited to 500 square feet and must typically be created within the existing or proposed primary dwelling.

Practical Size Ranges

Although San Diego may allow a 1,200-square-foot ADU, many real-world detached ADUs are smaller because of cost, setbacks, utilities, privacy, height, and backyard geometry.

• Studio ADUs: roughly 300 to 500 square feet
• One-bedroom ADUs: roughly 500 to 750 square feet
• Two-bedroom ADUs: roughly 800 to 1,000 square feet

Smaller ADUs are often easier to place within setback constraints and can still qualify for the state-law protections that matter most on constrained lots.

Site-specific factors such as setback requirements and parking exemptions near transit can significantly affect how large an ADU can realistically be.

Frequently Asked Questions

What is the maximum ADU size in San Diego?

For most new attached or detached ADUs, the maximum size is generally 1,200 square feet.

Can an ADU be larger than 1,200 square feet?

Existing structure conversions may follow different rules from new detached construction.

What is the 800-square-foot ADU rule?

California law protects one ADU of up to 800 square feet under certain conditions.

How large can a JADU be?

JADUs are generally limited to 500 square feet.

Related Research

• San Diego ADU Setback Rules Explained
• Do ADUs Need Parking Near Transit?

Source Notes

This article summarizes San Diego and California ADU size concepts for early feasibility research.

Informational only. Not legal advice, permit approval, architectural, engineering, or investment advice.