ADU Rules in San Diego: Size, Setbacks, Parking and Cost

If your property is inside the City of San Diego, you can generally build one detached ADU, one converted ADU and one junior ADU on a single-family lot. Standard ADUs run from 150 to 1,200 square feet. If your property is in unincorporated San Diego County, the rules differ on size, setbacks and parking. Which jurisdiction you sit in is the first question to answer, and it changes almost everything after it.

We are real estate advisors, not builders or architects, so this guide covers the rules as they affect buyers, sellers and owners deciding whether an ADU belongs in their plans. Always confirm specifics with the permitting agency before you design anything.

Chelsea Almario Real Estate Advisor

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TL;DR

City vs. County matters most. The City of San Diego and unincorporated County areas publish separate ADU rules, with different size caps, setbacks and parking requirements.
City size range: standard ADUs are 150 to 1,200 square feet, and JADUs are 150 to 500 square feet, per the City’s information bulletin.
Fire zones change the setback. High and Very High Fire Hazard Severity Zones require at least 4-foot side and rear setbacks in the City, and fire officials can ask for more.
Budget for surprises. The San Diego Housing Commission recommends a 15% to 20% contingency, because site conditions drive overruns.
Buying or selling with an ADU? Permit status and what is already on the lot affect value and offer strategy, so ask before you write an offer.

First, find out which rules apply to your lot
What you can build: types and unit counts
Size limits: City vs. County
Setbacks, height and fire safety
Parking
The ADU Home Density Bonus Program
Permits, plans and fees
What an ADU costs in San Diego
Financing and the grant question
Common mistakes to avoid
Our perspective: ADUs and your home’s value
Thinking about buying or selling a home with an ADU?
FAQ
Sources

First, find out which rules apply to your lot

San Diego County is a patchwork. The City of San Diego, Chula Vista, La Mesa, Coronado and the unincorporated areas each run their own planning departments. California state law sets a floor for what every city must allow, and local governments add their own details on top.

For most of our clients in Hillcrest, North Park, South Park, University Heights, Normal Heights and Kensington, the City of San Diego rules apply. For homes in unincorporated communities, the County’s Planning & Development Services rules apply instead. If you are looking at Chula Vista or La Mesa, check that city’s own ADU page, because this guide covers only the City and the County.

The quickest check is the parcel’s address on the jurisdiction’s website. If you are buying, ask your agent to confirm it before you tour a house on the strength of its ADU potential.

What you can build: types and unit counts

The City’s Information Bulletin 400 defines an ADU as an attached or detached dwelling of 1,200 square feet or less with complete independent living facilities, on a lot with a proposed or existing home. A junior ADU (JADU) is smaller and sits inside the existing house.

There are three common types:

Detached ADU: a separate small home in the backyard or side yard, often the most expensive and the most flexible.
Attached ADU: an addition that shares a wall with the main house.
Converted ADU: a garage, basement or other existing space turned into a living unit.

After the City Council’s 2025 reform package, the City clarified that one JADU, one converted ADU and one detached ADU are allowed on lots with an existing or proposed single-family home. That same package also implemented a state law letting ADUs be divided into condominium units that can be sold separately from the main house, which matters to anyone thinking about resale.

Multi-family properties follow different counts. The County’s handbook notes that state law allows multi-family properties a maximum of two detached ADUs, so check the rules for your specific building type.

Size limits: City vs. County

This is where the two jurisdictions diverge most visibly.

	City of San Diego	Unincorporated County
Standard ADU size	150 to 1,200 sq ft	Detached up to 1,200 sq ft
JADU size	150 to 500 sq ft	Up to 500 sq ft
Attached ADU cap	Within the 1,200 sq ft maximum	Up to 50% of the primary home’s floor area, with exceptions up to 850 or 1,000 sq ft (if more than one bedroom), never above 1,200 sq ft
Conversions	Not subject to the 1,200 sq ft cap	Permitted accessory structures can be converted without standard size, height or setback rules, if fire safety is met

Sources: the City’s August 2025 ADU bulletin and the County’s ADU and JADU handbook. The JADU figure for the County reflects the state-law limit, so confirm it with County planners for your parcel.

The practical takeaway: a conversion is the lightest lift on size rules. A new attached unit on a small home in the County can hit the 50% cap quickly, which is why the exceptions for 850 and 1,000 square feet matter.

Setbacks, height and fire safety

Setbacks decide whether an ADU fits on your lot at all.

In the City, an ADU can generally encroach into side and rear setbacks, subject to building and fire code. Two situations trigger a minimum 4-foot interior side and rear setback: ADUs taller than 16 feet next to residential property, and ADUs in Very High Fire Hazard Severity Zones. The City’s 2025 reforms extended the 4-foot rule to High as well as Very High zones and let the Fire Code Official require a greater setback to meet the California Fire Code.

In the County, new ADUs generally need 4-foot side and rear setbacks.

Fire is the issue buyers often overlook. Canyon-edge and hillside lots in neighborhoods like Kensington, Mission Hills and the eastern edges of the County can fall in fire severity zones. Planning & Development Services of San Diego County encourages applicants to contact their local fire department during the pre-design phase, before paying for plans. We agree. A fire official’s answer can change the footprint, the access road requirement and the budget.

Parking

Parking is another City vs. County split. In the City, parking is generally not required for an ADU, with exceptions in coastal areas. In unincorporated County areas, parking is generally required, subject to exemptions. That is the City’s information bulletin and the County handbook speaking, and the exemptions turn on details like distance to transit, so confirm yours.

The City’s density bonus program is a separate track. The 2025 reforms require one off-street parking space for each affordable and bonus ADU built outside a Transit Priority Area. If you are only building a standard ADU, the bonus program rules do not apply.

The ADU Home Density Bonus Program

The City runs an optional program that lets owners build additional “bonus” ADUs in exchange for including affordable ones. Within Transit Priority Areas, the City’s ADU program page describes one bonus ADU for every affordable ADU, with no cap on the total.

The 2025 reforms tightened the program:

It is prohibited in the lowest-density single-family zones (several RS-1 zones), except in highest- and high-resource areas away from open space and parks.
Lots in High or Very High fire zones must front an improved public street with at least two evacuation routes.
Bonus and affordable ADUs under 750 square feet now carry a community enhancement fee, which funds neighborhood infrastructure.

This program is mostly for investors and owners of larger lots. A typical homeowner adding one backyard unit can ignore it.

Permits, plans and fees

In the City, ADUs and JADUs need a building permit, plans and forms, and an online application. The fees are listed in the City’s Information Bulletin 501 and related schedules, and they vary with the proposal. Check IB-400 for the list of forms and the application path.

In unincorporated areas, the County’s building permit fee schedule (PDS-613) lists plan review and permit fees based on square footage, with some Over-The-Counter review options. Look at the current County fee schedule rather than relying on a blog’s figure, since it is updated each fiscal year.

A sequence that works for most owners:

Confirm the jurisdiction and zoning for the parcel.
Ask the fire department about access and setbacks.
Check utilities: sewer or septic capacity, water, electrical panel.
Get a site survey, especially on slopes.
Choose between permit-ready plans and custom design.
Submit the application and pay fees.
Build, inspect and finalize before renting or occupying.

What an ADU costs in San Diego

Cost is the question behind most ADU searches, and the honest answer is a range that depends on the site. We do not publish a single figure, because the public sources that matter point to the same lesson: the lot matters more than the square footage.

The San Diego Housing Commission’s ADU Pilot Program report gives a useful frame, though it reflects 2021 costs:

Soft costs, including contingency, ran roughly 20% to 30% of total construction cost. Hard costs were about 70% to 80%.
The Commission recommends a 15% to 20% contingency reserve.
A 224-square-foot traditional studio was estimated at $521 per square foot, compared with $285 per square foot for a 1,199-square-foot three-bedroom unit. Smaller units cost more per square foot.

The County’s ADU handbook adds several drivers:

New construction typically costs 125% more than converting an existing structure in good condition.
Larger units cost more overall but generally less per square foot.
Separately metered water, electrical or gas service costs more than tying into existing lines.
Hillside or sloped construction adds significant cost.
New detached ADUs are required to have solar systems.

Because construction costs have moved since 2021, treat the Housing Commission’s numbers as proportions, not a current quote. Get at least two or three written bids from licensed contractors for your lot before you decide.

Financing and the grant question

Many readers ask about a $40,000 California ADU grant. We recommend confirming its current status directly with the state before you plan around it. This overview of California ADU financing walks through where the state grant and local assistance stand and what to count before you build.

For City homeowners with qualifying lower incomes, the San Diego Housing Commission runs an ADU Finance Program with affordability covenants attached. Ask the Commission for current terms and limits.

If you are buying a home and plan to build, talk to a lender before you make an offer. Renovation or construction financing that evaluates the home’s value after the ADU is complete usually fits better than a standard mortgage. If you are a first-time buyer, our guides to down payment assistance in San Diego and first-time home buying cover what comes first.

Common mistakes to avoid

The County handbook and the Housing Commission’s report agree on where projects go wrong:

Skipping early feasibility work. Check eligibility, zoning, setbacks, topography, fire requirements, septic feasibility and utility capacity before design.
Underestimating total cost. Include permit, design, construction and site-improvement costs, not only the build.
No contingency. Site-specific surprises such as soil, utilities and slopes are where overruns come from.
Unvetted contractors. Check that a contractor’s license is active.
Building without a permit. Unpermitted units can surface at inspection, in a lender’s appraisal or during escrow.

The Housing Commission also advises assembling experienced design, permitting and construction professionals and using permit-ready plans where possible.

Our perspective: ADUs and your home’s value

We see ADUs from two sides: buyers who want one, and sellers who already have one.

For buyers, an ADU is a floor-plan question and a financing question. Multi-generational households, couples who want a rental unit to offset a mortgage, and families who want a home office all look at ADUs differently. Before an offer, we ask whether the existing unit is permitted, whether it has its own meters, and whether the lot could take a second one. In a low-inventory market, a property with a legal ADU can be compared against others only with careful comps, because few sold homes match it.

For sellers, we are cautious. An ADU can help your home stand out, but a buyer’s appraiser and lender will look for permits. If you have a garage conversion that was never permitted, talk to us and to the City before you list. It is easier to disclose and plan around a known issue than to renegotiate in escrow. If you are building one before selling, run the numbers on what you would actually net, not just what the unit adds to the list price.

For LGBTQ+ couples and families, an ADU can also be a way to share a property in a household structure that does not fit a standard mortgage, such as friends or chosen family buying together. If that is your situation, the questions about joint title and lenders come before the ADU ones, and our LGBTQ+ home buyer guide covers them.

— Chelsea Almario

Thinking about buying or selling a home with an ADU?

We can pull comps for properties with legal ADUs, check the permit history before you write an offer, and help you decide what to do about an unpermitted unit before you list. To see what your home would sell for today, start with a home valuation. If you are buying, book a free 20-minute consultation and tell us whether an ADU is a must-have or a nice extra.

FAQ

What are the ADU rules in San Diego right now?

In the City, a single-family lot can have one JADU, one converted ADU and one detached ADU. Standard ADUs run 150 to 1,200 square feet, and JADUs 150 to 500. Unincorporated County areas have their own size, setback and parking rules, so confirm which jurisdiction applies first.

What is the maximum ADU size in San Diego County?

In unincorporated areas, a detached ADU can reach 1,200 square feet. Attached ADUs are generally capped at 50% of the primary home’s floor area, with exceptions for units up to 850 or 1,000 square feet, and never above 1,200 square feet.

Do I need parking for an ADU in San Diego?

In the City, parking is generally not required, with exceptions in coastal areas. In unincorporated County areas, parking is generally required unless an exemption applies. Bonus ADUs outside Transit Priority Areas need one off-street space.

How much does it cost to build an ADU in San Diego?

It depends on the site, the size and whether you convert or build new. Smaller units cost more per square foot, hillsides and new utility meters add cost, and the Housing Commission suggests a 15% to 20% contingency. Get written bids on your own lot.