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Foundations

Every ADU sits on something, and the choice affects cost, schedule and what your engineer will sign. Slab-on-grade is common for site-built units. Factory-built units often sit on piers, screw piles or a perimeter foundation designed for the unit. The companies here make pier and pile systems. Your engineer and building department decide what is allowed on your soil, so bring them in before you pick a product.

Order is editorial. Descriptions come from each company's own website, last checked October 2026. A listing is not an endorsement. How we work.

Diamond Pier

Structurally rated, no-dig footings for decks, porches and posts, with versions for larger projects.

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A wooden house frame going up behind a construction fence
Design and build

Prefab vs site-built ADUs

Factory-built units can be faster and sometimes cheaper. The savings depend on your lot, your city and what the quote leaves out.

3 min read · 4 sources
A small modern cabin with large glass windows among trees
Site work

Utilities, sewer and power

Connecting an ADU can cost a few thousand dollars or more than the unit. Here is how to find out which before you sign.

3 min read · 3 sources
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