Straight answer: in Southern California in 2026, a garage-conversion ADU typically runs $90,000–$175,000, an attached ADU $175,000–$380,000, and a detached ADU $200,000–$400,000+. Below is the honest breakdown — including the soft costs most builders don’t mention until you’re already committed.
ADU Construction Costs by Type (Southern California, 2026)
| ADU type | Typical total | Per sq ft |
|---|---|---|
| Junior ADU (up to 500 sq ft) | $60,000–$175,000 | $200–$350 |
| Garage conversion | $90,000–$175,000 | $150–$350 |
| Attached ADU | $175,000–$380,000 | $260–$450 |
| Detached ADU | $200,000–$400,000+ | $270–$500 |
| Prefab / modular (all-in) | $120,000–$300,000 | $250–$450 |
Detached ADU Cost by Size (Los Angeles area, 2026)
| Size | Typical total |
|---|---|
| 400–500 sq ft | $150,000–$200,000 |
| 600–750 sq ft | $200,000–$280,000 |
| 800–1,000 sq ft | $250,000–$340,000 |
| 1,000–1,200 sq ft | $300,000–$400,000 |
Soft Costs: Budget $15,000–$35,000 on Top
Design ($8K–$20K), engineering ($1.5K–$4K), plan check ($2K–$6K), permits ($3K–$8K), and survey/soils ($2K–$5K) add up fast. Utility hookups — sewer, water, electrical runs, panel upgrades — can add another $0–$20,000. One bright spot: staying under 750 sq ft typically waives $5,000–$15,000 in impact fees.
What Moves Your Price Up or Down
- Up: hillside or tight-access lots, panel upgrades ($3K–$12K alone), high-end finishes, structural surprises in garage conversions.
- Down: simple rectangular designs, keeping plumbing near existing lines, stock finishes, building under 750 sq ft.
- County premium: LA County runs roughly 15–25% above the state average; Orange County 10–20%; Riverside comes in near or just below average.
Get an Exact Number for Your Property
Ranges are useful; a real bid is better. ADU Cities builds across Los Angeles, Orange, Riverside, and San Bernardino counties. Call 310.880.9718 for a free consultation — we’ll walk your lot and give you a real number, not a range.
Figures reflect 2026 Southern California builder and industry ranges; your project may vary. Always get a written, itemized bid.