800 sq ft ADUs
How much does an 800 sq ft ADU cost in 2026?
800 sq ft is the workhorse size — comfortable for a one-bedroom-plus-office or a tight two-bedroom layout, and the size that California state law uses to define the ADU impact-fee waiver threshold (units ≤750 sq ft are exempt). The 2026 range: $120,000–$320,000 nationally; $200,000–$320,000 in Southern California specifically.
Typical 2026 all-in cost
$120,000 – $320,000
150–400 $/sq ft · 800 sq ft, mid-finish, slab foundation.
Kellow Feb 2026; DQB Jan 2026.
Kellow Construction's published California ranges (Feb 2026): $120,000–$240,000 for 800 sq ft. DQB's SoCal data (Jan 2026): $200,000–$320,000 for the same size with the SoCal premium baked in. Per-sqft costs land around $250–$400.
By type and tier
| Type or tier | Range | Notes |
|---|---|---|
| Garage conversion (800 sf, rare) | $200K–$280K Bay Area | |
| Attached ADU (800 sf) | $200K–$300K SoCal | |
| Detached ADU (800 sf) | $200K–$320K SoCal; $350K–$480K Bay Area |
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 |
|---|---|---|
| Design & permits | 6–8% | |
| Site work | 5–8% | |
| Foundation | 8–10% | |
| Framing & shell | 15–18% | |
| Roofing | 5–6% | |
| Plumbing | 8–10% | |
| Electrical | 6–8% | |
| HVAC | 3–5% | |
| Insulation & drywall | 6–8% | |
| Kitchen | 8–12% | |
| Bathroom | 5–8% | |
| Flooring | 3–5% | |
| Windows & doors | 4–6% | |
| Exterior siding | 5–7% | |
| Utilities to ADU | 4–6% | |
| Contingency | 10% |
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.
800 sq ft ADUs FAQ
- Is an 800 sq ft ADU subject to impact fees?
- Yes — California state law waives impact fees for ADUs under 750 sq ft. At 800 sq ft you pay proportional impact fees, which in major California metros can add $10,000–$25,000 to the project (Andalusia Drafting, 2026). For some homeowners, designing to 749 sq ft to capture the waiver is the right move.
- What's the per-sqft cost for an 800 sq ft ADU?
- $250–$400 per sq ft in California for mid-finish, moderate site complexity (DQB 2026). Premium finishes and difficult sites push toward $500/sf.
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