Site-Built vs Modular ADU: Cost, Quality and Timeline in the Bay Area

People use these terms loosely, so start with how public agencies define them. Alameda County's ADU Guidebook describes a site-built ADU as one that's designed for your property and built on it (often called stick-built). A prefab, panelized or modular ADU is built partly or mostly in a factory, shipped to your site and set on a foundation.

In California, a factory-built unit gets reviewed in two places. Its plans and factory work go through HCD's Factory-Built Housing program, and each unit has to carry an HCD insignia of approval before it ships. Your local building department still inspects the installation on site, and local zoning, setbacks, fire zone rules and design review still apply. Manufactured homes built to the federal HUD code can also count as ADUs, according to HCD's ADU Handbook.

Factory-built ADUs are often marketed with a unit price, and that number usually leaves out work you'll still pay for. Alameda County's guidebook says prefab ADUs often cost the same as site-built ADUs, and that transportation, crane, foundation and sales tax can add up to 20–40% of the total cost. It also notes some companies do offer prefab ADUs at lower price points, so get real quotes before you rule anything in or out.

A lot of the work is the same either way. Every new detached ADU needs a foundation, utility connections and permits, and most need solar under the California Energy Code (HCD notes manufactured homes are treated differently). That's why the regional planning figure of roughly $400 to $550 per square foot all-in from ABAG and Alameda County is a reasonable starting point whichever method you pick.

What about the research on factory savings? A 2019 McKinsey report found modular projects have a solid record of cutting schedules by 20–50%, but called cost savings "more the exception than the norm today." UC Berkeley's Terner Center found off-site methods could save up to 20% on construction for three- or four-story wood-frame apartment buildings, where the same module repeats many times. That kind of repetition is harder to get with one backyard unit.

Payment timing can differ too. In the multifamily projects Terner studied, factories generally expected about 50% of the module cost up front when the order was placed. Alameda County also notes prefab ADUs can be harder to permit and finance. If you're borrowing, ask your lender early how they handle a factory deposit.

In the Bay Area, this is usually where the decision gets made. A factory unit has to travel on public roads and get lifted into place, so your lot needs room for a truck and a crane, and the design has to work within what can be shipped and set. Alameda County suggests thinking early about how an ADU would work on your specific property, including slopes and obstacles for installation. Terner Center lists crane limitations and less flexibility on design changes among the challenges of off-site building.

Site-built wood framing doesn't carry those limits. Lumber can come through a side gate, and the floor plan can bend around a tree, a slope or an odd property line. A few of our projects show what that looks like:

• Bates, El Cerrito: a 710 sq ft L-shaped one-bedroom on a west-facing hillside, with a 400 sq ft deck looking out at the Bay.
• Arlington, Berkeley Hills: a 375 sq ft one-bedroom tucked into a corner of the yard, with a low flat roof so it doesn't block the main house's view of San Francisco.
• Ramona, Albany: a 475 sq ft L-shaped studio that hugs a corner of the lot where an old garage used to stand.

To be fair to factory builders, Alameda County points out that there isn't always a hard line between the two, and some prefab designs are highly customizable. If your lot is flat and open, a factory unit may fit fine. If it isn't, that usually settles the custom vs prefab ADU question for Bay Area homeowners.

Factory building's biggest advantage is time on site. Because modules can be built while the foundation goes in, Terner Center found off-site methods can cut construction time by 40–50% on multifamily projects, and McKinsey reports modular projects finishing 20–50% faster. HCD's factory-built housing pamphlet also lists year-round building without typical weather delays, a controlled environment and less material waste as benefits, with the caveat that they vary by project.

For a backyard ADU, though, much of the calendar isn't construction. Alameda County says most ADU projects take 12 to 18 months overall. Construction itself usually runs 6 to 12 months, and within that, site prep often takes 1 to 2 months and the foundation about a month. Its guidebook puts permitting at 1 to 6 months. Permits, site prep and the foundation happen on your lot no matter where the walls are built.

For comparison, most Type Five ADUs take about 10 to 12 months from the first design meeting to move-in: roughly a month of design, two to four months of permitting and about six months of construction. You can see what happens at each stage on our construction schedule page.

We build on site because most Bay Area backyards aren't flat, empty rectangles. Framing in place lets us work through a side gate, step a foundation into a hillside and adjust details as the house goes up.

Our design system and in-house permitting keep the process predictable, and you get a guaranteed budget for the whole project before construction starts. We act as your architect, permit expeditor and general contractor from first sketch to move-in.

If you're weighing prefab for a full-size house rather than an ADU, our Bay Area prefab home guide covers that side. For ADU budgets and how to pay for one, see our ADU cost and financing guide.