Prefab ADUs
What does a prefab ADU actually cost in 2026?
Prefab and modular ADUs are advertised at $80,000–$300,000 for the unit itself — 10–20% per-sqft savings on construction over site-built (LADU, Jan 2026). But the unit price is rarely the total. Site preparation, foundation, permits, utility connections, and finish work bring the all-in cost much closer to site-built. The honest 2026 range: $200,000–$450,000 all-in.
Typical 2026 all-in cost
$200,000 – $450,000
150–400 $/sq ft
LADU Jan 2026; BuildX Mar 2026.
LADU (Jan 2026) reports prefab unit pricing of $150–$300/sqft compared to $250–$400+/sqft for site-built — a real per-unit savings. But BuildX (Mar 2026) documents how the advertised number understates by $150,000–$250,000 because mandatory site costs aren't included. The true all-in for a $150,000 advertised modular unit lands at $350,000–$450,000 — the same as a site-built ADU.
By type and tier
| Type or tier | Range | Notes |
|---|---|---|
| Studio prefab unit (400–500 sf) | Unit $80K–$150K · All-in $180K–$280K | |
| 1BR prefab unit (500–800 sf) | Unit $150K–$300K · All-in $250K–$400K | |
| 2BR prefab unit (800–1,200 sf) | Unit $250K–$450K · All-in $350K–$550K |
What moves the price most
Crane access
Narrow streets, steep drives, and overhead power lines push crane costs up — possibly requiring traffic control, larger cranes, or police details. Five-figure crane bills are common on tight LA lots (LADU 2026).
Utility distance from main house
Long sewer runs, undersized water meters, and main panel upgrades add work. All-electric prefab models often need service capacity upgrades to support HVAC and appliances (LADU 2026).
Interior finish completion
Many modular units arrive with primer-only walls and shell-grade finishes. Painting, upgraded counters, fixtures, and trim can add $20K–$50K (BuildX 2026).
Payment terms
Most prefab manufacturers require full unit payment before placement. That's a large lump-sum cash outlay vs site-built's draw schedule tied to construction milestones (BuildX 2026).
Cost breakdown
Share of total mid-point cost by line item (industry rules-of-thumb; actual ratios shift with site complexity and finish level).
| Line item | Share | Amount |
|---|---|---|
| Prefab unit (factory price) | 40–55% | $80K–$300K |
| Site preparation | 5–10% | $10K–$30K |
| Foundation | 8–15% | $25K–$50K |
| Delivery + crane + set crew | 5–8% | Often $10K–$30K |
| Utility connections | 5–10% | $10K–$25K |
| Permits + engineering + Title 24 | 3–5% | $5K–$15K |
| Interior finish completion (often excluded) | 5–15% | Varies widely |
Want sharper numbers for your specific project?
Run the calculator with your inputs, or get matched with 2–3 local builders who can quote your lot directly.
Prefab ADUs FAQ
- Is prefab really cheaper than site-built?
- Per-sqft on the unit itself, yes — typically 10–20% less than site-built (LADU 2026). All-in once site prep, foundation, utility work, and finishes are added, the numbers usually converge. BuildX (2026) documents cases where a $150K advertised module became a $350K-$450K all-in project — identical to site-built for the same size.
- How fast is prefab installation?
- The factory build runs in parallel with on-site foundation and utility work, which compresses the calendar. LADU (Jan 2026) reports 2–6 months order-to-final for prefab vs 6–9+ months for site-built. Note that permitting, site prep, and inspections still happen at California's normal pace.
- What questions should I ask any prefab quote?
- Confirm in writing: unit spec and finishes; foundation cost; delivery and crane plan with route survey; utility connection allowances; permit handling and Title 24 documentation; interior finish completion or shell-only; payment terms and timing. If the answer to any of these is vague, treat the quote as incomplete.
Get sourced ADU cost updates monthly
No spam — just our quarterly re-validated cost ranges and any California state-law changes affecting your project.