Factory-built units · local site work

Prefab and Modular ADUs in Walnut Creek

A prefab ADU is built in a factory and set on a foundation poured on your lot. In the Bay Area, an 800 sq ft prefab runs about $300,000–$450,000+ all-in once foundation, crane, delivery, hookups and permits are counted (IMKAT, 2026). Factory "from" prices are not all-in.

Request a Quote Call (925) 709-5438

How it works

Factory unit plus local site work

Choose a modelFrom a manufacturer's catalog; some plans are pre-approved by Walnut Creek or the County.
Site design and permitSite plan, foundation, utilities; a city or County building permit is still required.
Foundation and utilitiesA local crew pours the foundation and brings sewer, water and power to the pad.
Delivery and setTruck and crane place the unit; the crew connects utilities and finishes seams.
Final inspectionSame sign-offs as a site-built ADU.

Price check

All-in vs base price

What you see	What it covers
Base or "from" price	The factory-built unit only
Installation price	Delivery, crane, set, connections (varies by vendor)
All-in price	Design, permits, factory build, delivery and installation
Not always included	Unusual site work: grading, retaining walls, long sewer runs, panel upgrades, tree work

Example: Abodu lists Walnut Creek pricing from $298,800 all-in, built from a $234,800 base home plus published installation (Abodu). We don't endorse any manufacturer; compare at least two all-in quotes.

Access

Crane access on hillside lots

Prefab depends on getting a truck close enough for a crane to reach the pad. Narrow streets, overhead lines, steep driveways and mature oaks can rule it out or require a larger crane and street closure. On many Orinda and Lafayette hillside lots, that access question is settled before anything else.

Site-built or panelized construction is the fallback when a full module can't be set.

The module rides in on a truck; the foundation, utilities and crane pad are local work.

Fit

Which projects suit prefab

Good fit	Harder fit
Flat or gently sloped backyard with street access	Steep lots, long driveways, tight side yards
A standard 1- or 2-bedroom layout	A custom shape to match the house or a view
Owners who want a fixed unit price early	Projects with big site-work unknowns
Timeline matters more than customization	Garage conversions and JADUs (not prefab projects)

Prefab or site-built? Get both priced.Get a Quote or call (925) 709-5438

FAQ

Prefab ADU questions

Is a prefab ADU cheaper than site-built?

Not always. The factory unit is only part of the cost; foundation, crane, delivery, utility hookups, permits and site prep are local work. IMKAT puts an 800 sq ft prefab at $300,000–$450,000+ all-in in the Bay Area, versus $360,000–$520,000+ for a site-built unit of the same size.

Do prefab ADUs need a permit?

Yes. A factory-built ADU still needs a city or County building permit, a foundation design, and utility sign-offs. Some vendors' plans are pre-approved by the County (PADU) or by Walnut Creek, which shortens review.

How long does a prefab ADU take?

Factory production can run alongside permitting, and the set itself takes a day or two, but foundation and utility work still happen on site. Total time is often shorter than site-built, but permits and site work set the pace.

Detached ADUs → · ADU cost → · Choosing a builder →

Next step

Check whether prefab works on your lot

Share your lot and goals. A local ADU builder can review feasibility and give you a quote.

Request a Quote Call (925) 709-5438