Do ADU / Garage Conversions Need A Geotechnical Soil Report?

Accessory Dwelling Unit (ADU) refers to a secondary dwelling unit found on the same lot as a primary unit (a home). Examples include house extensions, garages, and garage conversions, either attached to or detached from the primary unit.

If you live in the Los Angeles or Glendale areas, a critical part of the permitting process for ADUs is to obtain an approved geotechnical soil report. Learn whether all types of ADUs require a soil report in your area, what kinds of reports you may need, how much they cost, and whether you must meet other requirements before starting your project.


Why Soil Reports Are Essential

The primary goal of a geotechnical soil report is safety. Soil can behave in many unpredictable ways, making it essential to determine whether it is safe to build on.

Additionally, the Los Angeles metropolitan area is often subjected to extreme weather events and specific natural disasters, such as earthquakes or mudslides. As a result, local soil may expand, contract, or move, potentially damaging building foundations and causing buildings to collapse.

A soil report is essential to ensure your ADU won’t collapse after periods of heavy rain or during a natural disaster. It provides detailed information on what type of soil your property stands on and gives you suggestions on preparing your building project and reinforcing it against potential dangers.

Examples of potential dangers that soil testing and reporting can reveal include the following:

  • Fault evaluation and earthquake activity can reveal whether the property would be affected by an earthquake.
  • If the property is on a slope or a hillside, a report can tell whether the underlying soil is sufficiently stable for the desired construction.
  • During earthquakes and heavy weather events, the soil may be subjected to a phenomenon called liquefaction, which causes the previously solid ground to act like a viscous liquid and slide down. A report can reveal whether the property is at risk of becoming affected by liquefaction.

Depending on the report’s findings, your geotechnical firm may recommend safety modifications to your project, including:

  • Installing post-tensioned or deep foundations to mitigate the dangers of expansive soil.
  • Replacing unstable, disturbed, or otherwise dangerous soil with imported fill material.
  • Excavating part of the soil to flatten the slope.


Report Requirements

The requirements for an ADU permit application vary depending on your area of residence.

For instance, in the city of Los Angeles, a full or modified geotechnical soil report is typically only required for properties situated on hillsides with a slope. However, it is a good idea to order one even if your property rests on a flat surface.

In contrast, the city of Glendale requires a full or modified geotechnical soil report for all ADU projects, no matter whether the property is on a sloped hillside or a flat surface.

There are no requirement differences between the ADU types. In other words, it doesn’t matter if you’re building a granny flat, a garage, or a different kind of ADU; all have the same soil report requirements.


Full or Modified?

You may have noticed you have the option to submit either a full or a modified geotechnical soil report. But what, exactly, is the difference between the two?

A full geotechnical soil report is essentially a comprehensive investigation of your property’s soil. Soil engineers will visit the property, conduct visual inspections and determine whether more complex and involved geophysical testing is required. They will take soil samples from two locations: on the surface and beneath the surface.

The engineers then send these samples to a geotechnical engineering laboratory, analyzing every aspect of your soil and conducting various tests. One testing example is R-Value testing, which analyzes your soil’s response to vertical pressure and determines whether it can support heavy loads without excessively spreading. This testing procedure is essential to verify whether your soil can safely support the weight of your ADU.

On average, a full geotechnical soil report costs $7,000 to $9,000, taking between 3 weeks and 2 months to complete, depending on the complexity.

In contrast, a modified soil report isn’t as intensive. A soils engineer simply visits the property and makes a brief report based on observations and prior experience instead of direct sampling and analysis. A modified soil report can be completed much faster (1-2 weeks) and costs much less than a full report: $1,500 on average.

Typically, not every jurisdiction may accept modified soil reports for every type of project. Fortunately, Los Angeles and Glendale will accept modified soil reports for ADUs, such as garage conversion projects.

Trust AES, Your Local Geotechnical Engineering Experts

Based in Glendale, CA, Applied Earth Sciences (AES) has over 30 years of geotechnical engineering experience all over Southern California. We offer a complete suite of soil investigation and analysis services, ranging from environmental site assessments to methane mitigation.

Are you looking to obtain a geotechnical soil report for your ADU? We offer full and modified reports alongside our expertise and guidance to help you ensure the safety of your building project. For a quote or more information, call us today at (818) 552-6000 or contact us through our request form.

Scroll to Top