ADU builders
Soils report for an ADU: when it is required
By the HyreADU research desk. Last checked .
The short answer
A soils report for an ADU, also called a geotechnical report, tests the ground the foundation will sit on. Under California Residential Code R401.4, the building official may require a soil test where the soil is likely to be expansive, compressible or shifting. Many cities also require one in seismic or liquefaction zones.
When a city asks for one
The code leaves the call to the building official. Common triggers are clay soils that swell and shrink, fill dirt, steep slopes, and mapped liquefaction or landslide zones. Some cities publish a policy that says when they waive it, for example where reports from nearby lots already show the soil.
Where no test is required, the code lets the designer assume standard soil bearing values from a table instead.
What the report gives you
A geotechnical engineer drills or digs test holes, then reports the soil type, bearing capacity, groundwater and any expansion risk. The structural engineer uses it to design the foundation. Without it, a foundation may be over-built or under-built.
Why it is a budget line, not an extra
Ground conditions are the most common unknown in a new foundation. Ask your builder whether the bid assumes a soils report and who carries the risk if the ground is worse than assumed. Put the answer in the contract.
What to do next: read the questions to ask an ADU builder, check what decides ADU cost, then tell us about your project. Matching is not live yet, so we reply with information, not a quote.
Get ADU optionsQuestions
Does a garage conversion need a soils report?
Rarely, unless new footings are added. Ask your city.
Who does soils reports?
A licensed geotechnical or civil engineer. Check the license with the state board.
Can a soils report be reused?
Some cities accept reports from nearby lots. Ask the building department.
Does prefab skip the soils question?
No. A factory-built unit still sits on a site-built foundation.
Sources
- 2022 California Residential Code, R401.4 (soil tests). Checked October 27, 2026.
- California Board for Professional Engineers, Land Surveyors, and Geologists, license lookup. Checked October 27, 2026.
HyreADU is a publisher. It does not design, permit or build ADUs, has no partner builders, and gives no legal, tax or engineering advice. Rules vary by state and city; California rules are labeled as such.