Why soil conditions matter for your ADU
Planning to add an Accessory Dwelling Unit (ADU) to your California property? Ensuring a strong foundation starts with understanding the soil conditions on your site. A soils report is an essential step in the ADU construction process — here's what's involved.
What is included in a soils report?
A soils report (also called a geotechnical report) is a comprehensive assessment of the soil and subsurface conditions on your property. A typical report includes:
Site information — location and site details, previous land use, existing structures and utilities.
Soil composition — types and characteristics of soil, stability and compaction data.
Geotechnical recommendations — foundation design guidance, recommendations for excavation, grading, and compaction.
Earthquake and seismic analysis — evaluation of seismic hazards and recommendations for earthquake-resistant construction.
Boring and sampling data — soil sampling methods and results from borings at different depths.
Laboratory test results — soil density, moisture content, and bearing capacity from lab testing of field samples.
ADU requirements in California
ADUs have gained popularity as a way to add housing options in California, but there are specific requirements to follow:
- Zoning and permits — Check local regulations and obtain necessary permits from your city or county.
- Size and location — ADUs must generally be within an existing residential structure or in the rear yard; size limits vary for attached vs. detached units.
- Parking — Some areas require parking spaces, though recent legislation has relaxed requirements in many cases.
- Soils report — A soils report is commonly required to assess site feasibility for ADU construction.
Working on a project in Southern California? We can handle the engineering.
What makes a good soil test report?
A reliable soils report should be thorough (covering composition, stability, and bearing capacity), site-specific (with recommendations tailored to your property), and clearly presented (with supporting data, tables, and visuals).
Three items you'll always find in a soil test report:
- Soil composition — types of soil and their properties, including stability, compaction, and cohesion.
- Bearing capacity — the soil's ability to support your ADU foundation, with recommendations on foundation type.
- Site-specific recommendations — guidance for excavation, grading, compaction, and seismic conditions if applicable.
What does a soils report look like?
A soils report is a formal document prepared by a licensed geotechnical engineer. It typically includes a cover page with project details, a table of contents, a summary of key findings and recommendations, and detailed sections covering site information, soil composition, lab test results, and geotechnical recommendations.
The format varies, but the goal is always the same: give the design and construction team the information they need to build safely and correctly.
The soils report is the foundation of your foundation
A soils report for your ADU project is not just paperwork — it's the foundation of your foundation. Understanding the soil conditions on your property is key to making informed decisions and ensuring the success of your build. If you're considering an ADU in California, contact us to provide the geotechnical expertise to guide you through this critical phase.
Related Service
Soils Reports & Geotechnical Investigation
We handle this for projects across Southern California.
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ADU Engineering Reports: What Do You Actually Need?
Not every ADU needs a full soils report. Here's how to figure out what your jurisdiction actually requires — and what you can skip.
Do I Need a Soils Report for My ADU in San Diego?
The rules vary by city within San Diego County. Here's a jurisdiction-by-jurisdiction breakdown so you know what to expect before you start.
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