ADU Cost Calculator for 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.

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.

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.

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.

A typical Orange County ADU runs about 6–10+ months from permit clearance to certificate of occupancy, plus design and permitting time up front. Garage conversions are usually faster than ground-up detached units.

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.

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.

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.

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.