What Size ADU Can You Build in San Diego? Rules and Limitations

Adding an accessory dwelling unit (ADU) is one of the most effective ways to increase property value, create rental income, or house family members. But before you start designing, it’s important to understand how large your ADU can legally be in San Diego — and what limitations may apply to your property.

This guide breaks down the state and local regulations that determine ADU size and offers practical insight into how to make the most of your available space.

Statewide ADU Size Standards

California has standardized ADU laws to ensure homeowners across the state can build secondary units. Under these state laws:

• Detached ADUs may be up to 1,200 square feet, regardless of the size of the primary residence.
• Attached ADUs may be up to 50% of the primary home’s floor area or 1,200 square feet, whichever is smaller.
• Junior ADUs (JADUs), which must be built within the existing home, are limited to 500 square feet.

These rules establish a baseline. Cities and counties can add more specific requirements, as long as they don’t restrict what state law already allows.

San Diego’s Local ADU Size Rules

San Diego is considered one of the most ADU-friendly cities in California. The city’s regulations generally align with state standards but include a few important distinctions:

Detached ADUs

• Up to 1,200 square feet, or the maximum size allowed by zoning and lot coverage limits — whichever is greater.
• Typically allowed to reach two stories, subject to height and setback rules.

Attached ADUs

• Up to 1,200 square feet, or 50% of the main home’s floor area.
• Must share a wall with the existing home.

Multiple ADUs on a Single Lot
Properties located in Transit Priority Areas (TPAs) or certain multi-family zones may qualify for two or more ADUs on a single lot. Multi-family properties can often add both conversion and detached ADUs under recent state housing laws.

Setbacks and Height Limits

• Minimum 4-foot side and rear setbacks
• Height limits typically range from 16 to 30 feet, depending on zoning and lot size

Zoning and Lot Size Considerations

Your property’s zoning designation determines the total number and type of ADUs you can build:

• Single-family zones (RS) generally allow one ADU and one JADU.
• Multi-family zones (RM) may allow multiple ADUs, including conversions of existing non-livable space such as garages or storage rooms.
• Lot coverage limits usually range from 40% to 60% of total lot area, depending on zoning.

Confirming your zoning map or having an ADU specialist review your parcel can help you understand the exact capacity and restrictions for your property.

How to Maximize ADU Space

Even with regulations in place, design choices can help you make the most of your square footage:

• Build up instead of out to preserve yard space and meet lot coverage limits.
• Consider lofts or vaulted ceilings to create an open feel.
• Explore garage or basement conversions for additional livable space.
• Work with an experienced ADU planner who understands San Diego’s zoning overlays, height limits, and parking exemptions.

The Bottom Line

Most San Diego homeowners can expect to build an ADU between 800 and 1,200 square feet, but exact limits depend on your zoning, lot size, and whether the unit is attached or detached. Because both state and city rules evolve frequently, consulting a local ADU expert ensures your project stays compliant and efficient from the start.

Plan Your ADU with Confidence

At ADU Geeks, we help homeowners throughout San Diego navigate ADU design, permitting, and construction coordination. Our team can quickly determine what’s possible for your property and guide you through each step of the process.

Schedule a free consultation today to learn what you can build on your lot.