Free tool · ADU Cost Calculator
General-B License #1107538 · 37 OC cities & communities · 4.8★ from 35+ homeowners
ADU Cost Calculator for Orange County.
Calculate your ADU project cost in 60 seconds. Calibrated against 2026 OC bids, cross-checked by Gemini, follow-up by Avorino.
Avorino design · 600 sq ft · 1 bed
An original Avorino design drawn for answers like yours, not a unit we have built yet.
Cost bands last verified against Orange County market data in .
The guide
What an Orange County ADU costs, and how this estimate works.
Everything behind the number in one place: Avorino's published cost bands, how the calculator reaches your range, and the questions homeowners ask most.
How much does an ADU cost in Orange County?
Avorino prices most 2026 Orange County ADU projects at $135K–$220K+ for a Junior ADU, $85K–$170K+ for a garage conversion, and $245K–$440K+ for a detached build, all-in, design through certificate of occupancy. This free ADU cost calculator returns a range calibrated to your city, size, and finish in about a minute. These ranges have no separate soft-cost line: design, structural engineering, and city permit and plan-check fees are already inside the number, which is where construction-only quotes typically grow after signing.
Run your numbersAvorino's all-in Orange County ADU cost bands by type, 2026
- Junior ADU (JADU)
- $135K–$220K+
- Garage conversion
- $85K–$170K+
- Attached ADU
- $200K–$350K+
- Detached ADU
- $245K–$440K+
How to estimate an ADU cost in Orange County
Use the free ADU cost calculator to produce a 2026 Orange County cost range for your own property in about a minute.
- 01
Enter the property
Give the address, city and ZIP of the property. Pick the address from the list and the city and ZIP fill in from the county record. City matters because permit fees and plan-check practice differ across the 37 Orange County jurisdictions.
- 02
Choose the ADU type and size
Pick detached, attached, garage conversion, or above-garage, and set the target square footage. Type sets the baseline; a conversion inherits a slab and shell that a detached build has to create.
- 03
Set the finish level and systems
Choose bedrooms and bathrooms, ceiling height and terrain, kitchen and bathroom finish, flooring, HVAC and utility configuration. Finish level is the largest swing after size and type.
- 04
Add optional scope
Add a rooftop solar array if you want one, so the range reflects the whole project rather than the building alone.
- 05
Read the calibrated range
The tool returns an all-in range calibrated against active 2026 Orange County bids, covering design, engineering, permits and construction. It is a planning range, not a quote.
What an Orange County ADU really costs.
What it costs
How much does it cost to build an ADU in Orange County?
Most Orange County ADUs fall in these ranges: a detached ADU runs about $245K–$440K+, an attached ADU $200K–$350K+, a junior ADU $135K–$220K+, and a garage conversion $85K–$170K+. Your number depends on square footage, ADU type, site and utility work, finish level, and city fees — which is exactly what the estimator above calibrates for.
What drives ADU cost the most?
Square footage and ADU type set the baseline. After that, the biggest swings come from site conditions (grading, soils, utility runs), foundation type, finish level, and city-specific permit fees. Detached units cost more than conversions because they add a new foundation and independent utilities.
Does the price include design, engineering, and permits?
Yes. Avorino works on fixed-price design-build contracts that include architectural plans, structural engineering, Title 24 energy calculations, city permits and plan-check, and construction — so the range reflects a turnkey project, not just the build.
How long does an ADU take to build?
A typical detached or attached ADU in Orange County takes about 5–8+ months from contract to certificate of occupancy, with design and permits included. Junior ADUs and garage conversions are usually faster, typically 4–6+ months.
How much does an ADU cost in California compared to Orange County?
Statewide averages hide huge spread, inland markets can build for less, while coastal Southern California carries premium labor, fees, and finish expectations. In Orange County specifically, most detached ADUs land in the $245K–$440K+ range and garage conversions in the $85K–$170K+ range. This estimator prices Orange County, not a state average.
How much does a garage conversion cost in Orange County?
Typically $85K–$170K+ turnkey for a full garage-to-ADU conversion, the most affordable path to a permitted unit because the slab and shell already exist. A Junior ADU conversion inside the primary home runs $135K–$220K+. The estimator does not have a Junior ADU mode — its conversion mode is sized for a garage conversion, so use the published Junior ADU band above and talk to us for a figure on your specific layout.
About the estimate
Is this estimate a quote?
No — it's a calibrated estimate, not a contract price. The range is built from active Orange County 2026 bids and cross-checked by AI, but a final fixed price is only confirmed after a site walk and engineered drawings. Avorino does that walk and issues the fixed-price design-build contract that replaces this estimate.
Can I see the rental return and financing too?
Yes. Pair this with the ADU ROI Calculator to project rental income and payback period, and review the ADU financing options to see how to fund the build.
How accurate is an ADU cost calculator?
Accuracy comes from the data behind it. Generic national calculators average whole-state costs; this one is calibrated to active 2026 Orange County bids and refreshed against market data, then AI cross-checks the range for your inputs. Treat it as a planning range, the fixed price comes from a site walk and engineered drawings, and Avorino contracts fixed-price so the number you sign is the number you build at.
Your range, then the next question.
Model the rent, price a conversion, or size the loan on the same project. When you're ready, Avorino confirms the number for your lot.
Questions
Quick answers
How much does an attached ADU cost in Orange County?
We price most attached ADU projects in Orange County at $200K to $350K+. This all-in range includes design, structural engineering, city permit and plan-check fees, and construction.
What is the cost of a junior ADU in Orange County?
We price most Junior ADU projects in Orange County at $135K to $220K+. This covers a conversion inside the primary home, from design through certificate of occupancy.
Why does a detached ADU cost more than a garage conversion?
We see detached units cost more than conversions because they add a new foundation and independent utilities. A garage conversion inherits a slab and shell that a detached build has to create.
How does this ADU cost calculator calculate an estimate?
We calibrate the calculator against active 2026 Orange County bids. You enter your address, ADU type, size, and finish level to generate an all-in planning range for your city.