Short answer: probably yes, but it depends on the city
San Diego County has 18 incorporated cities plus the unincorporated county — and each one sets its own requirements for ADU soils reports. Some require them for every detached ADU. Some waive them on flat lots under a certain size. Some have over-the-counter programs that bypass the normal plan check process entirely.
This post covers the jurisdictions where we work most frequently. If your city isn't listed, call their building department directly and ask: "Do you require a geotechnical investigation report for a detached ADU on a flat lot?" That question gets a straight answer.
City of San Diego
The City of San Diego almost always requires a soils report for detached ADUs with new foundations. The Development Services Department (DSD) will flag the requirement during the permit application review.
The exception: the City has an ADU Standard Plan program where certain pre-approved ADU designs on qualifying flat lots can proceed without a new geotechnical report. To qualify, the lot must have no significant slope, no known soil hazards, and the proposed foundation must match the pre-approved design. If you're using a standard plan ADU, ask your permit expediter or the DSD counter whether your lot qualifies.
For hillside lots in San Diego — and the city has many, from Clairemont to Point Loma to SDSU-adjacent neighborhoods — a soils report is mandatory, full stop. Slope stability analysis is also required if any grading is involved.
Plan check at DSD currently runs 6–10 weeks for first-round review of residential projects including ADUs. Budget accordingly.
Unincorporated San Diego County
The County of San Diego (Department of Planning & Development Services) requires soils reports for detached ADUs in most cases. The county covers large swaths of East County, North County, and backcountry areas including Alpine, Lakeside, Ramona, Valley Center, and Fallbrook.
County lots tend to be larger, more rural, and more likely to have slope conditions, which makes the soils report genuinely important — not just a permit formality. Many county parcels have expansive soils, fill areas from previous grading, or slope instability that needs to be evaluated before a new structure goes in.
County plan check for residential ADUs typically runs 4–8 weeks. The County has been working to improve turnaround, but complex sites take longer.
Chula Vista
Chula Vista generally requires soils reports for detached ADUs with new foundations. The Building Division follows a similar standard to the City of San Diego — new foundation construction requires geotechnical backing.
Western Chula Vista (west of I-805) is mostly flat and well-studied. Eastern Chula Vista — Eastlake, Otay Ranch, Rolling Hills Ranch — involves more complex terrain with cut-and-fill pads, retaining walls, and slope conditions from master-planned development. Even in these areas, the original subdivision soils reports often cover individual lots, which can sometimes satisfy the requirement for an ADU. We can check what's on file before you commission a new investigation.
Working on a project in Southern California? We can handle the engineering.
El Cajon, Santee, La Mesa, and East County Cities
These cities are generally consistent with San Diego in requiring soils reports for detached ADUs. El Cajon and La Mesa have older housing stock with mixed foundation quality, and the building departments are appropriately cautious about new construction without geotechnical backing.
Santee has seen significant ADU activity and the building department is experienced with the process. Plan check is typically faster than the City of San Diego — often 3–5 weeks for residential ADUs.
One note for East County: the soils in this region include more expansive clay and decomposed granite than coastal San Diego. Expansion index testing is always required and sometimes drives more robust foundation recommendations than a coastal site of similar size.
Escondido, Poway, Santee, and North County Inland
These jurisdictions generally require soils reports for new detached ADU foundations. Escondido has a mix of flat and hilly terrain; Poway is known for its rocky, DG-heavy soils and occasional hillside lots that require slope stability analysis.
Poway's building department is known for being thorough — plan check comments on geotechnical reports are common, and they frequently ask for clarification on foundation depth, compaction specs, and lateral earth pressure for any walls. Getting the report right the first time matters more here than in some other cities.
Encinitas, Carlsbad, Oceanside, and Coastal North County
Coastal North County cities generally require soils reports for detached ADUs. These areas have unique soil conditions: sandy coastal soils, bluff-edge lots with erosion concerns, and some areas with shallow bedrock.
Encinitas Building Division has been active on ADU permitting and is familiar with the process. Carlsbad's Building Department is similarly efficient. Both cities' plan check for residential ADUs typically runs 4–6 weeks.
If your ADU is near the coast or on a bluff, expect additional scrutiny on slope stability and foundation depth — coastal erosion and bluff retreat are active concerns that the plan checker will flag.
What if there's already a soils report on file?
This is worth checking before you commission new work. Many properties in San Diego County — especially in newer subdivisions — already have soils reports on file with the building department from prior construction. If a report covers your lot and the scope is compatible with an ADU, the plan checker may accept it with an update letter from a licensed geotechnical engineer.
To check: call the building department and give them your APN (Assessor Parcel Number). Ask if there's a geotechnical report on file for the property. If yes, get a copy and bring it to us — we'll tell you whether it covers an ADU foundation or if new investigation is needed.
Reusing a compatible existing report can save $2,000–$5,000 and 3–4 weeks off your schedule. It's worth the 10-minute phone call to find out.
The bottom line
For detached ADUs on flat lots in San Diego County: plan on needing a soils report in most cities. Budget $2,500–$5,000 and 3–4 weeks from authorization to stamped report.
For attached ADUs (garage conversion, addition): soils reports are less commonly required. Check with your specific city, especially if the existing foundation is old or the structure will bear new loads.
For any ADU on a hillside: soils report plus slope stability analysis, no exceptions. Start this process before you finalize the design — the geotechnical findings may change your foundation type or building location.
Questions about your specific lot? Call us. We know San Diego County's jurisdictions well and can tell you exactly what you're likely to need before you spend a dollar on engineering.
Related Service
Soils Reports & Geotechnical Investigation
We handle this for projects across Southern California.
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