ADU Cost by Type and Size (Orange County)
A detached ADU in Orange County typically runs $96,700–$214,800 for 600 square feet and a garage conversion $48,900–$108,100. Attached and junior ADUs are quoted by the contractor. Builders price from published construction cost guides adjusted for Orange County labor; GetProEstimate shows you that number for your lot first, then one licensed Orange County contractor quotes to the same scope you saw.
Detached ADU, 400 sq ft
$78,000–$173,100
Studio. Standard to high-end finishes.
Detached ADU, 600 sq ft
$96,700–$214,800
One bedroom. Standard to high-end finishes.
Detached ADU, 800 sq ft
$115,300–$256,200
Two bedroom. Standard to high-end finishes.
Garage Conversion, 400 sq ft
$48,900–$108,100
With a kitchen and bath. Standard to high-end finishes.
Source: the construction cost guides contractors bid from, adjusted for Orange County labor. Low end = standard finish, high end = high-end finish.
What Drives the Price in Orange County
Type: a detached new build costs the most, a garage conversion the least per square foot.
Size: 750 sq ft is the line for impact fees, 500 sq ft for school fees.
Utility runs from the main house, and whether the sewer line or electrical panel needs an upgrade.
Coastal zone review in beach cities.
Site work on hillside or narrow lots.
Finishes.
Your city’s plan-check fees.
Orange County is California’s third-busiest ADU county, behind Los Angeles and San Diego: 8,288 ADU building permits from 2023 to 2025. Its cities issued 8,031 of them, led by Fullerton (1,294, with 589 in 2025 alone), Garden Grove (1,265), Anaheim (1,219), Santa Ana (653) and Westminster (613). Buena Park more than tripled, from 50 in 2023 to 179 in 2025. South County builds far fewer: Irvine issued 8 in 2025 and Dana Point 13. Source: California HCD Housing Element Annual Progress Reports, Table A2.
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Frequently Asked Questions
How much does an ADU cost in Orange County?
$96,700 to $214,800 for a 600 square foot detached ADU, $78,000 to $173,100 at 400 square feet and $115,300 to $256,200 at 800, from standard to high-end finishes; a 400 square foot garage conversion runs $48,900 to $108,100. Attached and junior ADUs are quoted by the contractor. The estimate prices your lot from your address and your answers.
Which Orange County cities build the most ADUs?
By the state’s permit records for 2023 to 2025: Fullerton 1,294, Garden Grove 1,265, Anaheim 1,219, Santa Ana 653 and Westminster 613. North and central Orange County cities with older single-family lots lead; South County cities issue far fewer.
Can my HOA stop me from building an ADU in Orange County?
No. California law voids HOA rules that prohibit or unreasonably restrict an ADU or junior ADU on a single-family lot. The HOA can still set reasonable rules, like design standards, as long as they don’t effectively block it.
Do I pay impact fees on an ADU in Orange County?
Not on an ADU under 750 square feet; state law bars impact fees below that size, and larger ADUs pay fees in proportion to the main house. School district fees apply only above 500 square feet. Plan-check and permit fees still apply in every city.
How long does an ADU permit take in Orange County?
State law gives the city 60 days to approve or deny a complete ADU application. Beach cities can add time where a lot is in the coastal zone, and a second round of plan corrections is the most common delay.
Why does the estimate differ from a builder’s quote?
The estimate is the published cost-guide price adjusted for Orange County labor; a quote is one builder’s price for one design. The licensed Orange County contractor you work with through GetProEstimate quotes to the same scope you saw.