If you own a home in San Diego, an accessory dwelling unit (ADU) can add rental income, house family, and raise your property value. The first question every homeowner asks is simple: how much will it cost?
The honest answer is that ADU cost in San Diego typically ranges from $40,000 for a junior ADU to $300,000 or more for a detached unit. Where your project lands depends on the type of ADU, its size, your site conditions, and the finishes you choose.
This guide breaks down real cost ranges, what drives your price up or down, the extra costs homeowners often miss, and how to plan a realistic budget.
ADU Cost in San Diego by Type
The type of ADU you build is the biggest factor in your final price. Here are the typical ranges for San Diego in 2026:
| ADU Type | Estimated Cost Range |
| Detached ADU | $150,000 to $300,000+ |
| Attached ADU | $100,000 to $200,000 |
| Garage Conversion ADU | $80,000 to $150,000 |
| Junior ADU (JADU) | $40,000 to $80,000 |
These are planning ranges, not fixed quotes. Every lot is different, so a detailed estimate for your property is always the best next step.
Detached ADU
A detached ADU is a standalone backyard cottage. It is the most popular choice in San Diego because it offers the most privacy and the most design freedom. It also costs the most, since you are building a complete structure from the foundation up, including framing, roofing, plumbing, electrical, and finishes.
Attached ADU
An attached ADU is an addition that shares at least one wall with your main house. It costs less than a detached unit because it uses part of your existing structure, but you still need new foundation work, framing, and utility tie-ins.
Garage Conversion ADU
Converting a garage is often the most cost-effective way to add a legal unit, because the walls, slab, and roof already exist. Costs rise if the slab needs leveling, insulation upgrades, or new plumbing lines. For a deeper look, read our guide to garage conversion ADU costs, permits, and what to expect.
Junior ADU (JADU)
A JADU is created inside your existing home, usually from a bedroom or other interior space, and is capped at 500 square feet under California law. Because it uses existing space, it is the lowest-cost route.
What Does an ADU Cost Per Square Foot in San Diego?
Many homeowners budget by square foot. For new construction in San Diego, a general planning range is roughly $250 to $450 per square foot, depending on design complexity and finish level. Smaller units often cost more per square foot because kitchens, bathrooms, and utility connections cost about the same no matter how small the unit is.
For example, a 600 square foot detached ADU can land anywhere from $150,000 to $270,000 depending on site work and finishes. Treat per-square-foot figures as a starting point and not a final price.
What Drives the Cost of Building an ADU?
Two ADUs of the same size can have very different price tags. These are the main factors:
Size and layout. More square footage means more materials and labor. A studio costs less than a two-bedroom, and a simple rectangular footprint costs less than one with complex rooflines.
Site conditions. Sloped lots, tight access for equipment, existing trees, or poor soil can add to grading, foundation, and excavation costs. San Diego has many hillside lots where this matters.
Utility connections. Running water, sewer, electrical, and gas lines from your main house to a detached unit can be a major line item, especially if the distance is long or the existing service needs upgrading.
Finishes and fixtures. Cabinetry, countertops, flooring, appliances, and lighting range from budget friendly to high end. If you are comparing material costs, our guide to flooring options for San Diego homes can help you plan.
Design and engineering. Architectural plans, structural engineering, energy compliance documents, and sometimes soils reports are all required before construction starts.
Costs Homeowners Often Forget to Budget For
The construction price is only part of the picture. Plan for these as well:
- Architectural and engineering fees for plans and structural calculations
- Permit and plan check fees charged by the city
- Utility and connection fees, which may apply depending on the unit type
- Site preparation, such as demolition, grading, or tree removal
- Landscaping and hardscape to restore your yard after construction
- Fire sprinklers or electrical panel upgrades if your property requires them
- A contingency fund of about 10 to 15 percent for surprises
California law limits or waives certain impact fees for smaller ADUs, which can save thousands of dollars. Rules change, so confirm current requirements with your builder. Our article on San Diego ADU rules in 2026 explains what every homeowner should know before starting.
How Long Does It Take, and Why That Affects Cost
Time is money on any build. Most San Diego ADU projects take 6 to 12 months from first consultation to move-in. Permitting usually takes 2 to 4 months, and construction typically runs 3 to 6 months once permits are approved.
Delays in design revisions, plan check corrections, or material orders can increase your total cost, especially if you are paying rent elsewhere or carrying a construction loan. Working with an experienced local builder helps keep your schedule on track. If you are also planning other upgrades, see how long a home remodel takes in San Diego.
Is Building an ADU Worth the Cost?
For many San Diego homeowners, yes. A well-built ADU can generate roughly $1,500 to $3,000 per month in rental income depending on location and size. That income can offset a large share of your mortgage or construction loan payment.
An ADU also adds usable square footage and long-term value to your property. Even if you never rent it, it can serve as space for aging parents, adult children, or a home office. For a full overview of planning, permits, and timelines, read our complete ADU construction guide for San Diego homeowners.
How to Keep Your ADU Cost Under Control
You do not have to sacrifice quality to stay on budget. These strategies help:
- Start with a clear budget and goal. Know whether you are building for rental income, family, or resale.
- Choose a simple design. Rectangular footprints and standard rooflines cost less to build.
- Consider a garage conversion or attached unit. Using existing structures can cut thousands from your total.
- Make finish decisions early. Late changes lead to change orders and delays.
- Hire a licensed, insured contractor. Cheap bids that skip permits or cut corners usually cost more in the long run.
- Get a line-item estimate. Transparent pricing helps you compare bids fairly.
If you are thinking bigger, you may also want to explore SB 9 eligibility requirements to see whether your lot qualifies for additional development options.
Build Your ADU With XLNC Builder
XLNC Builder is a licensed, bonded, and insured San Diego contractor (CSLB Lic. #1153455) that specializes in custom ADU construction. We handle design, permitting, and building so you have one team from first sketch to final walkthrough. You can also browse our ADU projects and learn more about our custom ADU construction services.
Ready for a real number for your property? Request a free quote or call us at (619) 836-0099.
Frequently Asked Questions
What is the average cost to build an ADU in San Diego?
Most San Diego ADUs cost between $80,000 and $300,000. Garage conversions and junior ADUs sit at the lower end, while detached units can reach $300,000 or more depending on size, site conditions, and finishes.
Is a garage conversion cheaper than building a detached ADU?
Yes, in most cases. A garage conversion typically costs $80,000 to $150,000 because the foundation, walls, and roof already exist. A detached ADU often costs $150,000 to $300,000 or more since the whole structure is built new.
Are there permit and impact fees for ADUs in San Diego?
Yes, you will pay permit and plan check fees. State law limits or waives certain impact fees for smaller ADUs, which can reduce your total cost. Fee amounts depend on your unit size and type, so ask your builder for a current estimate.
How long does it take to build an ADU in San Diego?
Most projects take 6 to 12 months overall. Permitting usually takes 2 to 4 months, and construction generally takes 3 to 6 months once permits are issued.
Can an ADU pay for itself through rental income?
Many homeowners earn about $1,500 to $3,000 per month from a San Diego ADU, depending on location and size. That can cover a large part of your loan payment, and the time it takes to recoup your investment varies by project cost and rental rates.
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