What Does a Prefab ADU Actually Cost?
If you're a California homeowner considering a backyard ADU, cost is probably your first question. The short answer: prefab ADU units typically range from $130,000 to $250,000 depending on size and finishes — but the total project cost includes more than just the unit itself.
Let's break down every cost component so you can plan accurately.
Unit Price: The Factory-Built Structure
The unit price covers everything built in the factory: framing, insulation, electrical, plumbing, HVAC, interior finishes, appliances, windows, roofing, and exterior cladding.
For reference, EVO ADU offers five models ranging from 350 to 800 square feet:
- 350 sq ft studio — Starting at $135,800
- 430 sq ft studio — Starting at $147,920
- 560 sq ft 1-bedroom — Starting at $169,680
- 715 sq ft 2-bedroom — Starting at $193,050
- 800 sq ft 2-bedroom — Starting at $228,000
All prices are for the factory-built unit plus installation. Pricing subject to change.
Site Work & Foundation
This is where costs vary the most. Site work includes:
Grading and excavation — Preparing your lot for the foundation. Flat lots cost less; sloped lots require more work. Budget $5,000–$15,000 depending on conditions.
Foundation — Most prefab ADUs use a concrete slab or pier foundation. Typical cost: $8,000–$20,000 depending on soil conditions and local requirements.
Utility connections — Running water, sewer, and electrical from your main house to the ADU site. Distance matters — longer runs cost more. Budget $10,000–$25,000.
Permitting & Design
California's ADU-friendly laws have streamlined permitting significantly. Most prefab ADU companies use pre-engineered, state-approved plan sets that simplify local approval.
Typical permitting costs range from $2,000 to $8,000 depending on your city and county. Some jurisdictions have waived or reduced ADU permit fees.
Installation
Installation covers crane delivery, setting the unit on the foundation, and connecting all utilities. For a quality prefab ADU, installation typically runs $15,000–$30,000.
Total Project Cost: Putting It All Together
For a mid-range prefab ADU in California, here's a realistic total budget:
| Component | Typical Range |
|---|---|
| Unit (factory-built) | $135,000–$230,000 |
| Site work & foundation | $15,000–$40,000 |
| Utility connections | $10,000–$25,000 |
| Permitting | $2,000–$8,000 |
| Installation | $15,000–$30,000 |
| Total | $177,000–$333,000 |
Prefab vs. Site-Built: The Cost Comparison
Traditional site-built ADUs in California typically cost $250–$400 per square foot all-in. A 600 sq ft site-built ADU could easily run $200,000–$300,000 or more — and take 12–18 months.
Prefab ADUs offer several cost advantages:
Factory efficiency — Controlled production environments reduce waste and labor costs. No weather delays, no site theft, no coordination headaches.
Speed — Factory production takes 6–8 weeks. Total project timeline is typically 4–6 months vs. 12–18 months for site-built. Less time means less carrying cost on your construction loan.
Predictable pricing — With a prefab unit, the biggest cost component is locked in upfront. Site-built projects are notorious for change orders and cost overruns.
Financing Options
Most California homeowners finance their ADU through one of these options:
Home equity — HELOCs and home equity loans leverage your existing equity. With California home values, many homeowners have significant equity available.
Construction loans — Purpose-built for ADU projects, these convert to permanent financing after completion.
Cash-out refinance — If rates are favorable, refinancing your primary mortgage can free up funds for the ADU.
Monthly payments for a prefab ADU can start around $1,053/month depending on loan terms and credit qualification.
All financing subject to credit approval. Rates and terms vary by lender.
How to Get an Accurate Quote
Every property is different, so the best way to understand your total cost is:
1. Choose your model — Start with the unit that fits your needs and budget
2. Check site eligibility — Verify your property meets zoning and setback requirements
3. Get a site evaluation — A site visit identifies specific conditions that affect cost
4. Receive your project quote — A complete quote covering unit, site work, and installation
The more you know upfront, the fewer surprises down the road.