An accessory dwelling unit is one of the highest-value home projects a San Diego homeowner can take on, whether the goal is rental income, space for family, or simply adding square footage without moving. It's also one of the widest-ranging in cost. Two homeowners can both say they're "building an ADU" and end up with quotes $200,000 apart, depending entirely on which type of ADU they're actually building.
Garage Conversion vs. Detached Build: The Real Cost Difference
The single biggest cost driver in any ADU project is whether you're converting an existing structure or building a new one from the ground up.
| ADU Type | Typical Total Cost |
|---|---|
| Garage conversion | $100,000 to $200,000+ |
| Above-garage ADU (new second story) | $200,000 to $320,000+ |
| Detached ADU (new build) | $250,000 to $450,000+ |
A garage conversion is almost always the least expensive path, since it reuses an existing foundation, roof, and at least some framing, which can reduce foundation and framing costs by $40,000 to $60,000 compared to building the same square footage from scratch. A detached new build has no existing structure to work with, so every part of the project, foundation, framing, roofing, utilities, starts from zero.
Cost Per Square Foot
Most San Diego ADU projects run $350 to $500 per square foot all-in for a turnkey build, with detached units sometimes running higher, $375 to $600+ per square foot, depending on finish level and site conditions. Garage conversions tend to land toward the lower end of that range on a per-square-foot basis, again because the existing structure absorbs some of the cost.
Soil conditions, utility access (how far the ADU sits from existing water, sewer, and electrical connections), and finish level move the final number more than square footage alone. Two ADUs the same size can differ by six figures based on these factors, which is exactly why a site visit and a real bid matter more than a generic price-per-square-foot estimate.
The Soft Costs Most Estimates Leave Out
Construction is only part of the total. Design, permitting, and fees commonly add another $20,000 to $40,000 on top of the construction number, and it's easy for a rough estimate to quietly leave these out:
- Design and architectural costs: commonly 6% to 9% of total project cost, roughly $24,000 to $36,000 on a typical $400,000 detached ADU
- Permits and city fees: commonly 5% to 9% of total costs, including plan check, building permit, and utility capacity fees. City of San Diego permit fees alone commonly run $13 to $28 per square foot
- School impact fees: required at permit issuance, commonly $3.79 to $6.56 per square foot depending on the school district, working out to roughly $4,000 to $8,000 on a typical-sized ADU
Ask any contractor bidding your project whether their quoted number includes these soft costs or only covers construction. A bid that looks meaningfully lower than others sometimes reflects a narrower scope, not a better price.
Permit Timeline Also Affects Your Budget
San Diego has adopted the state's ministerial ADU review process, meaning a complete application is generally reviewed within 60 days, and is automatically deemed approved if the city doesn't respond within that window. Using a pre-approved plan template, where available, can shave additional weeks off the process. From first submittal to final permit, a straightforward ADU project generally takes three to four months total, with more complex lots sometimes extending to five or six months.
Timeline matters for your budget too. A longer permitting or construction timeline means more months of holding costs, and in 2026's tighter labor market, longer project timelines are more common than they used to be. Building in a realistic timeline expectation upfront, rather than assuming the fastest-case scenario, helps avoid a budget surprise later.
Getting an Accurate Number for Your Property
Every figure in this guide is a general range, not a quote. Lot slope, existing utility locations, soil type, and how much of an existing garage or structure can realistically be reused all move the real number for a specific property. The only reliable way to get an accurate cost is a bid based on your actual lot, ideally more than one.
Posting your ADU project on Wrkbid lets you collect bids from multiple general contractors already working on ADU projects in San Diego County, so you can compare real numbers for your specific property instead of guessing from an average.
Frequently Asked Questions
How much does a garage conversion ADU cost in San Diego?
Garage conversion ADUs commonly cost $100,000 to $200,000 or more, depending on size and finish level. They're typically the least expensive ADU option since they reuse an existing foundation and structure rather than building entirely new.
How much does a detached ADU cost to build in San Diego?
Detached, ground-up ADU builds commonly cost $250,000 to $450,000 or more. Because there's no existing structure to work with, every component, foundation, framing, roofing, and utility connections, is built new, which is the main reason detached ADUs cost significantly more than a garage conversion of similar size.
What's a realistic cost per square foot for an ADU in San Diego?
Most turnkey ADU projects run $350 to $500 per square foot all-in, with detached units sometimes running $375 to $600 or more per square foot depending on finish level and site conditions. Garage conversions tend to land toward the lower end of that range.
What soft costs should I budget for beyond construction?
Design and architectural fees (commonly 6% to 9% of total project cost), permits and city fees (commonly 5% to 9%), and school impact fees (commonly $3.79 to $6.56 per square foot) typically add another $20,000 to $40,000 on top of the construction cost itself. Confirm with your contractor whether their bid includes these or only covers construction.
Does a garage conversion really save that much money compared to a detached build?
Yes, often significantly. Reusing an existing foundation and structure can reduce foundation and framing costs alone by $40,000 to $60,000 compared to the same square footage built from scratch, on top of generally landing at a lower cost per square foot overall.
How long does it take to get an ADU permit in San Diego, and does that affect cost?
A complete ADU application is generally reviewed within 60 days under the state's ministerial review process, with automatic approval if the city doesn't respond in that window. Total time from first submittal to final permit commonly runs three to four months, sometimes five to six for more complex lots. A longer timeline means more months of holding costs, so it's worth factoring into your overall budget, not just the construction price.
Will 2026 material costs affect my ADU budget?
Likely somewhat. Tariffs on steel, aluminum, and copper have pushed construction input costs up industry-wide in 2026, which affects the materials used in ADU framing, electrical, and plumbing work. It's worth asking a contractor directly how current material pricing factored into their bid rather than assuming an older cost guide's numbers still hold exactly.
How can I get an accurate cost estimate for my specific property?
General cost ranges are a starting point, not a quote. Lot slope, soil conditions, existing utility access, and how much of an existing structure can be reused all move the real number significantly. Getting bids from more than one contractor who has actually seen your property is the most reliable way to know what your specific ADU will cost.