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Building an Accessory Dwelling Unit (ADU) in San Diego County has become one of the most popular ways for homeowners to add value, generate rental income, or create extra living space. But before starting your project, the most common question is: how much does it actually cost to build an ADU in San Diego in 2025?
The short answer is: it depends. Costs can vary widely depending on size, design, location, and construction method — but most projects in San Diego fall somewhere between $300 to $500 per square foot for a full, turnkey build. Below, we’ll break down the main cost factors, explore differences between detached, attached, conversion, and prefab ADUs, and walk through a real-world example to give you a realistic picture.
Several major factors influence how much your ADU will cost. The biggest driver is size — larger units naturally cost more overall, but the cost per square foot often decreases slightly as you go bigger because some fixed expenses (like permits and utility hookups) don’t scale linearly.
Site conditions are another big variable. Flat, accessible lots are far more affordable than hillside or irregular sites that require grading, retaining walls, or special foundations. Similarly, the distance to existing utilities matters. If your sewer or water lines are far from where you plan to build, expect extra trenching, new connections, or pump systems that can add thousands to the total.
Finish level also plays a big role. High-end fixtures, tile, cabinetry, and appliances can push costs up significantly compared to standard finishes. Labor and material prices fluctuate as well — San Diego’s strong construction market keeps demand (and prices) relatively high compared to other regions.
Finally, your construction approach matters. Hiring a licensed design-build firm typically provides a smoother process but comes with professional management costs. Acting as an owner-builder or managing subcontractors yourself can reduce expenses, but it requires time, coordination, and risk management. Always make sure you’re aware of permit and liability requirements if going that route.
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Detached ADUs, often called backyard cottages or guest houses, are standalone structures built separately from the main residence. Because they require their own foundation, framing, roofing, and full utility systems, they tend to be the most expensive type of ADU to build.
In 2025, detached ADUs in San Diego typically range between $350 to $600 per square foot, depending on size and finish level. Smaller units (around 400 to 600 square feet) often land on the higher end of that range due to fixed permitting and site costs, while larger units (800 to 1,200 square feet) can achieve better value per square foot. Lots that are flat with utilities nearby will generally be closer to the lower range, while hillside lots or those needing extensive grading, retaining walls, or drainage improvements can push costs upward.
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Attached ADUs are built as extensions of the main home — for example, adding a new wing or converting space adjacent to the primary structure. Because they can share walls, roofs, and utilities, they tend to cost less than detached ADUs.
In 2025, most attached ADUs in San Diego fall between $300 and $450 per square foot. However, integrating new and old structures can introduce complexity, such as reinforcing framing, waterproofing transitions, or ensuring the existing home meets current building codes. If your existing home needs upgrades to comply with energy or structural requirements, those added costs can narrow the savings gap compared to a detached unit.
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Conversion ADUs — such as garage conversions, basement units, or converted bonus rooms — are often the most cost-effective option because you’re reusing an existing structure. You avoid the expense of new foundations, framing, and roofing, which make up a large portion of construction costs.
Typical 2025 costs for conversion ADUs in San Diego range between $200 and $400 per square foot, though that can vary based on the condition of the existing structure. If the building needs new foundations, seismic upgrades, or full electrical and plumbing rework, that range can climb quickly. Keep in mind that even simple garage conversions must comply with current residential codes, which include insulation, egress windows, and fire safety requirements — so the project is more involved than just adding drywall and flooring.
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Prefab ADUs — short for prefabricated units — are built off-site in a factory and then delivered for installation on your property. This controlled construction process reduces labor costs, minimizes weather delays, and can make prefab ADUs more affordable than traditional builds, especially for smaller footprints.
In 2025, turnkey prefab ADUs in San Diego County typically range from about $180,000 to $300,000+, depending on size, finish level, and delivery logistics. However, homeowners should still account for on-site costs such as foundation, utilities, and permits — which can add $30,000–$70,000 or more, depending on accessibility and slope. Because the factory build and site prep happen at the same time, prefab ADUs offer predictability and speed, with many projects completed in 3–6 months compared to 9–12 months for traditional, site-built ADUs.
To illustrate, let’s look at a realistic mid-range example. Imagine building a 600-square-foot detached ADU on a flat lot in San Diego with easy access to utilities about 30 feet away. You choose mid-grade finishes — nice but not luxury — and hire a professional builder to handle everything from design to construction.
In this scenario, base construction might cost around $195,000, with an additional $60,000 for mechanical, plumbing, and electrical systems. Site work, grading, and trenching might add roughly $10,000, while permits, engineering, and plan checks could run around $15,000. Finally, adding a contingency of about 10 percent brings the total to roughly $340,000 all-in, or about $570 per square foot. If the site were even simpler or if you opted for more economical finishes, you might see totals closer to the $300–$400 per-square-foot range. Conversely, a hillside site or custom design could easily exceed $600 per square foot.
(These numbers are for illustrative purposes only and should not be taken as fixed costs. Every property and project is unique.)
In 2025, San Diego’s permitting and fee structure continues to vary depending on the size and location of your ADU. Permit and plan check costs generally range from $6,500 to $21,000, depending on complexity. Under California’s SB 13, impact fees are waived for ADUs that are 750 square feet or smaller, which can save thousands of dollars.
Some local school districts, however, still charge small development fees for units over 500 square feet, and utility connection or capacity fees may apply for larger ADUs. Always check the most recent fee schedule for your specific city or water district within San Diego County, as each jurisdiction may have slightly different rules.
Even in a high-cost market like San Diego, there are ways to make your ADU more affordable:
So, how much does it cost to build an ADU in San Diego County in 2025? Most homeowners can expect to spend somewhere between $200,000 and $400,000 for a smaller ADU, and $350,000 to $600,000 or more for a larger, high-quality detached build. The wide range reflects the many nuances of site conditions, finishes, and construction approaches. The best way to narrow down your budget is to get a professional feasibility assessment. That way, you’ll understand your zoning, lot constraints, and likely construction costs before spending thousands on design or permits.
Still curious? Explore these additional resources to help you plan, design, and build smarter.
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