ADU Cost in Walnut Creek and Contra Costa
Expect about $150,000–$250,000+ for a garage conversion and $250,000–$450,000+ for a new detached ADU in Contra Costa (Mazzamuto Construction, 2026). Junior ADUs start lower; hillside lots, long sewer runs and high-end finishes push numbers up. Real quotes can land below or above these ranges.
By type
Cost by ADU type
| ADU type | Typical size | Total cost range | Per sq ft (Bay Area) | Source |
|---|---|---|---|---|
| Junior ADU (JADU) | up to 500 sq ft, inside the home | $110,000–$180,000+ | — | Mazzamuto |
| Garage conversion | 350–600 sq ft | $150,000–$250,000+ | $250–$400+ | Mazzamuto · IMKAT |
| Attached ADU | up to 50% of the home | $220,000–$380,000+ | $400–$600+ | Mazzamuto · IMKAT |
| Detached ADU | 600–1,200 sq ft | $250,000–$450,000+ | $450–$700+ | Mazzamuto · IMKAT |
| Detached, 800 sq ft, all-in | 800 sq ft | $360,000–$520,000+ | — | IMKAT |
| Prefab, 800 sq ft, all-in | 800 sq ft | $300,000–$450,000+ | — | IMKAT |
| Prefab (Abodu, Walnut Creek) | varies by model | from $298,800 all-in | — | Abodu |
All figures are published by contractors or manufacturers, not by this site, and are labeled by source. Per-square-foot numbers cover construction; total ranges are closer to all-in.
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All-in
What "all-in" should include
An ADU bid that only prices the building leaves you to pay the rest. A complete number covers:
- Design and engineering: drawings, structural, Title 24 energy compliance; less with a pre-approved plan.
- Permits and plan check: city or County fees.
- Impact fees: none for ADUs under 750 sq ft; larger units pay in proportion to the main home (Gov. Code §66324). Lafayette cut its ADU impact fees 50% through 2026.
- Sewer: a detached ADU line joins the house lateral downstream of the main cleanout; Central San collects fees on these projects (Central San).
- Water and power: water provider requirements (EBMUD or Contra Costa Water District, by address) and often a panel or service upgrade.
- Site work: grading, retaining walls, drainage, trenching, tree protection; the hillside premium lives here.
- Finishes: cabinets, flooring, fixtures and appliances, usually as allowances.
Examples
Two worked examples
500 sq ft detached, flat lot
$225,000–$350,000+
500 sq ft × $450–$700+ per sq ft (IMKAT Bay Area construction rates). Assumes a short sewer run, adequate panel capacity and mid-range finishes. Under 750 sq ft, so no impact fees.
800 sq ft detached, all-in
$360,000–$520,000+
IMKAT's all-in Bay Area figure, typically 12–16 months to completion. At 800 sq ft the unit is above 750 sq ft, so proportional impact fees apply. A prefab of the same size: $300,000–$450,000+ all-in.
Examples use published per-square-foot and all-in figures; your lot, design and finish choices decide the real number.
Offsets
Rebates and fee relief
The SD7 ADU Accelerator offers up to $15,000 in participating cities, but Walnut Creek's share was fully claimed as of July 15, 2025. The state's $40,000 CalHFA grant is fully allocated. Fee relief is more reliable: no impact fees under 750 sq ft and Lafayette's 50% cut through 2026.
Comparing bids
How to compare ADU bids
- Put every bid on the same line items: design, permits, fees, utilities, site work, building, finishes.
- Convert allowances to dollars and compare them.
- Ask who carries the risk on sewer route, soils and hillside work.
- Check the CSLB license and keep the deposit at $1,000 or less.
By type
Cost detail by type
FAQ
How much does a 500 sq ft ADU cost in California?
How can I afford an ADU?
Is a garage conversion cheaper than a detached ADU?
Why do ADU quotes vary so much?
Next step
Get a number for your lot
Published ranges are a starting point. A site visit turns them into a real quote.