Across our completed Orange County projects, a garage conversion runs $90,000–$150,000 and takes about 4–5 months including permits; a new detached ADU runs $200,000–$450,000+ over 6–12 months; an attached ADU sits between, at $150,000–$280,000. This page publishes the line-item and city-level data behind those ranges — free to cite with attribution.
Published by L Square Design & Build (CSLB #1095476), an Orange County design-build contractor. The figures below reflect our completed-project experience — 200+ ADUs across Orange County and Los Angeles — published as a reference for homeowners, journalists and researchers. Last updated August 2026.
01The headline numbers (2026)
| ADU type | Typical all-in cost | $/sq ft | Typical timeline |
|---|---|---|---|
| Garage conversion | $90,000–$150,000 | $150–$300 | 4–5 months incl. permits |
| Detached ADU (new build) | $200,000–$450,000+ | $300–$450 | 6–12 months |
| Attached ADU | $150,000–$280,000 | $250–$400 | 5–9 months |
| Junior ADU (≤500 sq ft, within the home) | $60,000–$130,000 | varies | fastest permit path |
All-in means design, engineering, permits and construction — not financing, furniture, or major off-site work. Full context lives in our ADU cost guide.
02Where the money actually goes
For the most common project we build — a 2-car garage conversion of roughly 400–500 sq ft — here is the typical spend by line item:
| Line item | Typical range |
|---|---|
| Design, engineering & permits | $8K–$15K |
| Demolition & prep | $3K–$5K |
| Structural work | $15K–$25K |
| Electrical, plumbing & HVAC rough-in | $25K–$35K |
| Insulation & drywall | $8K–$12K |
| Interior finishes | $25K–$40K |
| Exterior & final | $8K–$15K |
Sum the low ends and the high ends and you get $92K–$147K — which is why our published range for a garage conversion is $90K–$150K. The number isn’t marketing; it’s arithmetic.
03The five costs homeowners don’t expect
- Slab replacement
Older garage slabs are often not thick or level enough for living space — the most common surprise in postwar cities like Santa Ana and Anaheim.
- Electrical panel upgrade
A maxed-out 100-amp panel usually needs an upgrade or subpanel before a new dwelling can hang off it.
- Sewer distance
Trenching cost scales with the run from the unit to the sewer tie-in — a back-of-lot unit can mean 60+ feet of trench.
- Fire separation
Required between an attached unit and the main house — invisible in inspiration photos, real in the budget.
- City plan-check rounds
Timelines vary sharply by city: the fastest OC counters approve in roughly 4–6 weeks; slower ones take 8–10+. Details in our timeline guide.
04What ADUs rent for across Orange County (2026)
| City | Typical monthly rent |
|---|---|
| Newport Beach | $3,000–$4,000 |
| Irvine | $2,500–$3,200 |
| Huntington Beach | $2,400–$3,000 |
| Costa Mesa | $2,200–$2,800 |
| Fullerton | $2,000–$2,500 |
| Anaheim | $1,800–$2,400 |
At these rents, most garage-conversion owners see payback in roughly 4–6 years, and a legal, permitted ADU typically adds 15–25% to property value. How to fund the build — including the FHA rule that lets lenders count 75% of expected ADU rent toward qualifying — is covered in our financing guide.
05Method note
Ranges reflect L Square Design & Build’s completed ADU projects (200+) across Orange County and Los Angeles as of 2026, plus plan-check experience in Irvine, Santa Ana, Costa Mesa, Huntington Beach, Anaheim, Tustin and Fullerton. Figures are typical ranges, not quotes; site conditions move real budgets. Reuse is permitted with attribution and a link to this page. Press and research enquiries: (949) 374-7980.