The question we get more than any other
Every week we talk to homeowners who are adding an ADU and aren't sure which engineering reports they actually need. Their contractor says one thing, their architect says another, and the building department website is ambiguous.
Here's the direct answer: it depends on jurisdiction, lot conditions, and ADU type — but the requirements are knowable, and they're usually less than people fear.
Attached ADUs: Usually the simplest case
An attached ADU — converted garage, addition to the main house, basement conversion — generally has the lowest engineering burden.
In most San Diego, Orange County, and LA cities, attached ADUs on flat lots with existing foundations do not require a soils report if the original building has one on file or if the lot conditions are straightforward. The plan checker reviews the existing foundation design and may accept it without new investigation.
- What attached ADUs do commonly require:
- Grading plan: Only if you're modifying site drainage, adding impervious area, or changing pad elevations. A simple garage conversion often doesn't need one.
- LID/WQMP: Required in San Diego if you're adding more than 500 SF of new impervious surface. Orange and LA counties have similar thresholds. A garage conversion that doesn't change drainage may be exempt.
- Structural analysis: Always — but that's your structural engineer's scope, not geotechnical.
Detached ADUs: More likely to need geotech
A detached ADU — separate structure in the backyard — involves new foundation construction. This is where soils reports become more likely.
San Diego City and County: Most detached ADUs require a soils report. The building department specifically looks for one during plan check. Exception: some over-the-counter ADU programs for small structures on straightforward lots may not require it. Ask at the counter before assuming.
Orange County cities: Requirements vary significantly by city. Irvine, Anaheim, and Newport Beach generally require soils reports for detached ADUs. Some smaller cities waive it for small structures (under 500–750 SF) on flat lots. Check the specific municipal code.
Los Angeles City: LA requires a soils report for most detached ADUs, especially in areas with expansive soils (most of the Valley, hillside areas). The Standard Plan ADU program has specific requirements — your plan checker can tell you if your project qualifies.
Riverside and San Bernardino: These jurisdictions generally require soils reports for new foundation construction, including ADUs. The soil conditions in these areas (expansive clay, caliche, fill areas) make it genuinely important — not just a permit checkbox.
Working on a project in Southern California? We can handle the engineering.
What about hillside ADUs?
If your lot has any significant slope, assume you need a soils report. Full stop.
Hillside ADUs require geotechnical investigation, slope stability analysis, and foundation recommendations that account for slope conditions. This isn't negotiable with building departments in any Southern California jurisdiction — and frankly, it's not something you want to skip. A detached structure on a slope without proper geotech is a liability, not just a permit problem.
Expect the soils report to cost more for hillside projects (more borings, deeper, slope stability analysis adds engineering time) and expect plan check to be more detailed.
The LID/WQMP question for ADUs
This one catches people off guard. Even if your ADU doesn't need a soils report, it may need a stormwater compliance document.
San Diego (LID report): Required if your ADU project creates or replaces more than 500 SF of impervious surface. That threshold is cumulative — roof + driveway + patio all count. A 1,000 SF detached ADU with a new concrete pad almost always triggers this.
Orange County and LA (WQMP): Similar thresholds apply. Most detached ADUs with new hardscape will trigger WQMP requirements if the total impervious area exceeds the jurisdiction's threshold.
The good news: for small residential ADUs, the LID/WQMP is usually straightforward. A small infiltration basin, permeable paving in the driveway, or bioretention planter can satisfy the requirement without a lot of cost.
The practical checklist
Before you start your ADU engineering scope, run through this:
- Attached or detached? Attached = lower engineering burden. Detached = likely need soils report.
- Flat lot or slope? Any meaningful slope = soils report + slope stability analysis.
- City/county? Requirements vary. We can tell you what your specific jurisdiction requires.
- How much new impervious area? Over ~500 SF new roof + hardscape usually triggers LID/WQMP.
- Existing soils report on file? Call the building department and ask. If a compatible report exists, it may cover your ADU too.
The fastest way to know for sure: call us with your address and ADU type. We'll tell you what your jurisdiction requires and what you can skip. It takes 10 minutes and it's free.
Related Service
Soils Reports & Geotechnical Investigation
We handle this for projects across Southern California.
More from the blog
LID vs. WQMP: Which Stormwater Report Does Your Project Need?
Two names, same underlying requirement — but they're not identical. Here's how to tell which one your project needs and why.
How Long Does a Soils Report Take in Southern California?
The honest timeline from site visit to stamped report — and what actually causes delays at each step.
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