Understanding San Diego County ADU Requirements & Guidelines

If you're a homeowner planning to build an accessory dwelling unit on your property, understanding the San Diego County ADU requirements is the first step to doing it legally and avoiding setbacks. ADUs are increasingly popular in California, but San Diego County has its own zoning rules, size limits, and permitting steps you'll need to follow. In this guide, we'll break down the San Diego County ADU guidelines in plain language so you know exactly what's allowed, what to prepare for, and how to move forward with your project.

Before you can build anything, you need to make sure your property qualifies. San Diego County doesn't allow ADUs on every lot. There are basic zoning and location rules in place. Your property must meet all of the following conditions:

• It is located in the unincorporated areas of San Diego County (not within city limits).
• It is zoned for residential use.
• It has an existing or proposed single-family home on the lot.

San Diego County allows several types of ADUs. The kind you choose will affect design, cost, and required permits. You need to know what you're building before you get started. The main ADU types allowed are:

• Detached ADU: A standalone unit, separate from the main home.
• Attached ADU: A unit connected to the main house but with its own living space.
• Conversion ADU: A unit created from existing space, like a garage or basement.
• Junior ADU (JADU): A unit built inside the existing home, limited to 500 square feet.

Size rules depend on the type of ADU you're building. These limits exist to make sure the unit fits the lot and complies with residential use. Here are the maximum sizes allowed, according to San Diego County's website:

• Detached ADU: Up to 1,200 square feet.
• Attached ADU: Up to 50% of the main home's area, maxing out at 1,200 square feet.
• Junior ADU: Maximum of 500 square feet.

The County controls where on your property you can place the ADU. These rules are based on zoning laws and are required for permit approval. Here are the general zoning and setback standards:

• Minimum Side and Rear Setbacks: Generally 4 feet. However, if you are in a "High Fire Hazard Severity Zone," fire codes may strictly enforce this or require even more distance to create "defensible space".
• Front Yard Setback Reduction: Allowed for ADUs under 800 square feet in some cases if there's no other place to build.
• Maximum Height for Detached ADUs: 25 feet.

Every ADU project in San Diego County requires a building permit. The application process is detailed, and missing documentation can cause delays or rejections. Getting your paperwork in order before submission saves you time and unnecessary fees. These are the standard items you'll need to submit:

• A complete site plan with property lines and ADU location.
• Floor plans and elevation drawings. (Consider professional if your ADU includes a full kitchen).
• Title 24 energy compliance documentation.
• Stormwater management plan (if applicable).
• Application forms and permit fees.

San Diego County does require parking for ADUs in most cases. However, there are a few exceptions depending on your situation. Make sure you know what applies to your lot before you start construction. The parking rules are:

• One space is required for each ADU.
• Tandem (front-to-back) and uncovered parking is allowed.
• Exception: No parking is required if the ADU is within ½ mile of public transit.
• Exception: No replacement parking is needed if you convert an existing garage.

If your property uses a septic system instead of public sewer, you'll need to go through additional steps. The County has strict rules to ensure your system can handle the added load from an ADU. Here's what you'll need to do:

• Check Capacity: A standard septic system is often limited to a certain number of bedrooms (often 6 total for the property).
• Tank Sizing: You may need a larger tank. For example, adding a 2-bedroom ADU usually requires a dedicated or upgraded septic tank capacity.
• Reserve Area: You must identify a "100% reserve area" on your lot—space kept empty in case the septic field needs to be replaced in the future.

Building an ADU is a significant investment, but it is one that pays off. By adding a rental unit, you create a steady stream of monthly income and increase the overall resale value of your property. While there are upfront costs, focusing on this long-term return on investment (ROI) is key to a successful project.

Historically, the County of San Diego operated a fee waiver program that removed plan check and permit fees to encourage development. This trial program expired on January 9, 2024.

• Permits Issued Before Jan 9, 2024: May still qualify for waivers.
• Current Projects: If you are applying now, you must pay the standard plan check and permit fees. The five-year window for free permits has closed.

It is critical not to confuse "San Diego County" rules with the "City of San Diego."

• The County: The fee waiver has ended.
• The City: The City of San Diego has its own separate incentives, such as the Affordable ADU Bonus Program. However, City rules are also evolving, with new updates proposed in March 2025 regarding single-family zones.

Always check if your property is in the unincorporated County or the City limits to know exactly which fee schedule applies to you.

Building an ADU in San Diego County is a smart financial move, but only if you follow the rules from the start. Skipping steps or misunderstanding the difference between City and County codes can cost you time and money.

If you're ready to build an ADU and want expert help with design, permits, or construction, 3 Day Kitchen & Bath is here for you. Our ADU contractors San Diego team is here to help get projects approved and completed the right way. Call us today to get started.

An ADU is a self-contained living unit with a complete kitchen and bathroom, legally allowed for full-time occupancy. A guest house may lack these features (often limited to a "wet bar" rather than a stove) and typically cannot be rented out independently.

Yes, in most cases, San Diego County allows one Detached ADU and one Junior ADU (JADU) on the same residential lot, provided you meet the size and setback requirements.

Not always. For JADUs and some attached ADUs, you can often share utilities. However, for detached ADUs or properties on septic, you may need upgrades or separate meters.

Yes, ADUs must match the style and appearance of the main home, including materials, roof slope, and exterior finishes, especially in areas with strict design review.

As of 2025, this is complex. The City of San Diego has adopted an ordinance allowing separate sales (condominiumization) starting August 2025. However, the County is still studying this program and has not yet finalized a law for unincorporated areas. Currently, in the County, you usually cannot sell the ADU separately.