Short answer. A prefab ADU in California runs $180,000 to $350,000 fully installed. The factory module is $90,000 to $200,000. Site work, foundation, utilities, and permits add the rest. Prefab makes sense on flat, accessible lots. Site-built makes sense on hillside, narrow, or unusual lots.
Prefab ADU total cost breakdown
| Line item | Typical range |
|---|---|
| Factory module (400 to 900 sq ft) | $90,000 to $200,000 |
| Foundation and slab | $15,000 to $30,000 |
| Crane and delivery | $8,000 to $20,000 |
| Utility connections | $10,000 to $25,000 |
| Site work and grading | $8,000 to $25,000 |
| Permits and design | $8,000 to $18,000 |
| Final connections and finish work | $5,000 to $15,000 |
Prefab vs site-built side by side
| Factor | Prefab | Site-built |
|---|---|---|
| Total cost (600 sq ft) | $200,000 to $300,000 | $240,000 to $360,000 |
| Total timeline | 4 to 7 months | 5 to 9 months |
| Design flexibility | Limited to factory options | Full custom |
| Best lot type | Flat, wide access, no overhead wires | Any lot |
| Finish quality | Consistent factory spec | Custom material selection |
| Two-story options | Limited | Standard |
When prefab makes sense
- Flat lot with 12-foot or wider access from the street to the install location.
- No overhead power lines that would block a crane lift.
- Standard one-story layout meets your needs.
- Schedule is the priority and you want predictable factory timing.
- You like a pre-set design and want a simpler decision process.
When site-built is the better path
- Hillside, sloped, or irregular lot.
- Narrow lot access where a crane and a truck cannot maneuver.
- Need a two-story ADU.
- Period or hillside home where the ADU should match a specific architectural style.
- Custom interior layouts not offered in factory catalogs.
For total ADU pricing, see ADU cost in Los Angeles and how much does an ADU cost in California. For permits, read ADU permit cost in LA. To start, see the ADU service page.
