A prefab ADU in California typically costs somewhere between about $150,000 and $400,000 all-in, with Bay Area projects running up to about $500,000, depending on size, region and, above all, your lot. Builders publish very different numbers because some quote only the factory unit while others quote turnkey. This guide shows the published ranges, breaks out every cost line, and explains what moves your number up or down.
Published California ADU cost ranges, 2026
| Region (all-in, backyard ADU) | Published range |
|---|---|
| Central Valley | $150,000 to $250,000 |
| Los Angeles | $200,000 to $350,000 |
| San Diego | $225,000 to $375,000 |
| Bay Area | $300,000 to $500,000 |
Source: Abodu, California ADU cost guide 2026. A second builder, Snap ADU, puts a turnkey 800 sq ft, two-bedroom prefab ADU at $280,000 to $430,000. Both are builders, so treat these as market ranges, not quotes.
Where the money goes
| Cost line | Published range | What drives it |
|---|---|---|
| Factory unit (prefab) | $150 to $250 per sq ft | Size, finishes, number of sections |
| Installation (prefab) | $50 to $100 per sq ft | Crane reach, access, transport distance and width |
| Design and planning | $5,000 to $15,000 | Site plan, engineering, surveys |
| Building permit | $2,000 to $8,000 | Your city’s fee schedule |
| Plan check | $1,000 to $3,000 | Your city’s fee schedule |
| Excavation and grading | $3,000 to $8,000 | Slope, soil, demolition |
| Total site prep for a prefab ADU | $30 to $80 per sq ft | Everything needed to make the lot ready for the set |
| Foundation | $8,000 to $20,000 | Type, slope, engineering |
| Electrical | $3,000 to $8,000 | Panel upgrade, separate meter, trench length |
| Plumbing (water and sewer) | $4,000 to $10,000 | Distance and slope to the sewer lateral; septic |
| HVAC | $5,000 to $12,000 | System type; often included in the factory unit |
Source for the ranges: Abodu, 2026. Site-built construction, by comparison, is published at $200 to $450 per sq ft before site work.
Costs state law limits or removes
- Impact fees: none for ADUs under 750 sq ft or JADUs; proportional above 750 sq ft.
- School fees: none for units of 500 sq ft or less.
- Parking: often none required, and a garage removed for the ADU doesn’t have to be replaced.
- Property tax: only the new ADU is assessed; your home keeps its existing base value (county assessor guidance).
Five things that blow up an ADU budget
- An electrical service upgrade with a long utility wait
- A long or uphill sewer run, or a septic system that needs review
- A hillside foundation
- A crane lift over the house with a long reach
- Comparing a factory-only price against a turnkey price
How to get a real number
Ask every builder for a turnkey price with a line-by-line list of what’s included and excluded: site work, foundation, utilities, crane, transport, permits, fees, steps and decks. If a line is “by owner,” price it yourself before you compare.
Frequently asked questions
Is a prefab ADU cheaper than building on site?
Sometimes. The factory price is more predictable and your yard is disrupted for less time, but site work, foundation and crane costs can close the gap. Compare turnkey prices.
What is the cheapest kind of ADU?
Usually a junior ADU or a garage conversion, because the structure already exists and they often avoid impact fees.
Does an ADU raise my property taxes?
Yes, but only by the assessed value of the new ADU. Your existing home is not reassessed.
Want a reality check on your budget before you call builders? Call (800) 806-7485. Next: How to finance an ADU in California.
By Matt Banes, iModular.com. California modular building specialist with 38 years in the industry. Last updated: October 7, 2026. Cost ranges are published market figures, not a quote.
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