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ADU vs. Garage Conversion
Everything you need to know
What's Actually Different Between the Two
The Decision Most Homeowners Get Wrong
If you already have a garage, converting it feels like the obvious shortcut — the structure, the slab, and often the roof are already there. And it is usually the cheaper starting point. But a garage conversion and a new detached ADU solve different problems, and the wrong choice can cost you parking, resale flexibility, or square footage you didn't realize you needed. A garage conversion works best when the garage is unused, structurally sound, and you're not relying on it for covered parking. A new detached ADU works best when you want a fully independent unit, don't want to give up your garage, or your garage isn't a good candidate for conversion in the first place.
Garage Conversion vs. New ADU: By the Numbers
Key Facts and Figures
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Garage conversions are generally the lowest-cost ADU option in California, typically $100,000–$250,000, since the structural shell already exists
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New detached site-built ADUs typically run $200–$450 per square foot, landing most complete projects between $150,000 and $400,000
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Prefab, factory-built detached units typically cost 15–25% less than a comparable site-built detached ADU
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A converted garage loses its covered parking function — some cities require a replacement parking space to be added elsewhere on the lot
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Impact fees are waived statewide for any ADU under 750 square feet, regardless of whether it's a conversion or new construction
THE TRADE-OFFS
What Actually Drives the Decision
Cost isn't the only variable — it's just the one that gets the most attention. A garage conversion can look cheaper on paper, but the existing structure often needs real work to function as living space: new foundation-level moisture barriers, insulation the garage never had, rerouted electrical, and plumbing run from scratch if there wasn't a bathroom before. Older garages may also have structural or ceiling-height issues that aren't obvious until construction starts, which is one of the more common sources of cost overruns on conversions.
A new detached ADU avoids most of that guesswork because it's a purpose-built living space from the foundation up — but it means starting from bare yard instead of an existing shell, which is why it typically costs more than a straightforward conversion. Parking is the other factor that gets overlooked: converting a garage removes covered parking from the property, and depending on the jurisdiction, that space may need to be replaced elsewhere on the lot to stay compliant.
How AmerADU Compares
Fixed, Factory-Built New Construction
✔ Detached, purpose-built units — no dependency on your existing garage's condition or layout
✔ Fixed starting price per model, with no surprise costs from hidden structural issues in an old garage
✔ Significantly under the $200–$450 per sq. ft. market average for site-built new construction
✔ Full living space from day one — proper insulation, plumbing, and electrical, not retrofitted into a converted shell
✔ Built in California to CALGreen and WUI fire-resistant standards
If your garage is in good condition and you don't need the parking, a conversion can be the faster, lower-cost path. If you want a fully independent unit without giving up your garage — or your garage isn't a strong candidate for conversion — a new AmerADU detached unit gives you a fixed price and a purpose-built space without the unknowns of retrofitting an old structure.
ADU VS. GARAGE CONVERSION — WHAT TO EXPECT
Garage conversions and new detached ADUs solve different problems, and the right one depends on what you're giving up. A conversion is usually the lower-cost path because the structural shell already exists, but it comes with parking trade-offs and a higher risk of hidden costs once construction opens up the existing structure. A new detached unit costs more up front but gives you a fully independent, purpose-built space with a fixed price and no dependency on your garage's condition. AmerADU's prefab detached models are built around that second path — giving you cost certainty without the guesswork of retrofitting an existing structure.
Frequently Asked
Common Questions About ADU vs. Garage Conversion
Usually, but not always. Garage conversions typically run $100,000–$250,000 because the foundation and structure already exist. But if the garage needs significant structural work, new insulation, or a full plumbing run, the cost gap between a conversion and a new detached unit can narrow quickly.
In most cases, yes — a converted garage no longer functions as covered parking. Some California cities require homeowners to replace that parking elsewhere on the lot, so it's worth checking local requirements before committing to a conversion.
Garage conversions generally qualify for the same ADU financing programs as new construction, including CalHFA and FHA Title 1 loans, since state law treats both as ADUs. Program-specific eligibility should still be confirmed with the lender.
It depends on the garage's existing condition. A straightforward conversion with a sound structure can move faster than new construction. But if the garage requires significant remediation, a prefab detached unit — built off-site while permitting proceeds — can end up being the faster path to a finished, move-in-ready space.