ADU Soils Reports: When Do You Need One?

Most homeowners assume a flat-looking backyard is ready for construction, but the ground beneath often tells a different story. Before designing your ADU, it’s important to know whether the soil can safely support a foundation. That’s where a soils report comes in. And depending on what the soils engineer finds, the next step may be overexcavation, which prepares the building pad to meet geotechnical standards.

This guide explains what a soils report actually does, when overexcavation becomes necessary, what each step costs, and how to time this work in the ADU design process so you avoid surprises.

What is a Soils Report?

A soils report is an analysis of soil conditions and how those would affect a building foundation. The licensed geotechnical engineer preparing the report is looking for problems that could affect the structure, such as loose soil, expansive clay, high moisture, or undocumented fill. If any of these are present, the report will specify what corrective steps are required to create a stable building pad. A soils report serves as an insurance policy to ensure your accessory unit is built safely, so it can be enjoyed for years to come.

Steps in a Soils Report

A soils report happens in two phases:

1. Initial Soil Investigation: The engineer visits the site, drills small borings, and analyzes samples. This produces the formal soils report with recommendations.
2. Field Verification During Construction: When the excavation crew digs for the actual foundation, the soils engineer performs on-site tests to confirm the real-world conditions match the report’s assumptions. If they don’t, adjustments may be required.

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How much does a soils test cost?

As of late 2025, soils reports in San Diego generally cost $6,000 to $7,000 or more depending on complexity and depth of investigation. It’s worth noting that we have seen a marked increase in the cost of soils reports in San Diego County in recent years. Engineers who perform these highly technical investigations are in high demand, and as a result, the market has seen price increases.

What is Overexcavation?

If the soils engineer finds that the upper layer of soil is loose, clay-heavy, too wet, or contains old fill, they may recommend “overexcavation.” Overexcavation is the corrective grading work the soils report may recommend. It involves removing the top layer of soil within the building pad, reconditioning it, and placing it back in compacted layers. The goal is to create a uniform, structurally reliable base for your ADU.

A simple way to picture it: You dig out the upper soil, mix and moisture-condition it, then compact it back down so the ground is consistent and strong enough to hold a foundation without future settlement.

The soils report diagnoses the condition of your soil. Overexcavation is the treatment plan.

Is overexcavation always required?

No. But it is common across San Diego because natural soils vary significantly from yard to yard, even in the same neighborhood. Some homes sit on excellent native soil; others sit on old fill, clay, or material that was never properly compacted. When a soils report identifies issues that could affect stability or drainage, the engineer may require removing and recompacting 18 inches of material or more. In some cases, deeper remediation may be needed.

We’ve done about 70 soils reports, and almost every one of them has required, at some level, an over-excavation.

Typical cost of overexcavation

Reprocessing about 18 inches of soil for an ADU building pad usually costs $3,000 to $4,000 depending on access and the amount of soil moved.

Cities generally defer to the soils engineer’s recommendations. If the soils report calls for overexcavation, you will be required to perform it as a condition of safe construction.

Do I need a soils report for my ADU?

There are several reasons you may need a soils report for your ADU project.

Required Due To Poor Soils Conditions

Another reason to obtain a soils report is if your lot has questionable soils conditions, such as expansive soil. SnapADU recommends obtaining a soils report even if not required, as results may call for remedial grading measures and/or more robust foundation parameters to ensure long-term stability. The downside of not having a soils report is that unknowns can arise during the foundation excavation, which could necessitate modifications to the design, increased costs and delays in the project.​​

Required Due To Slope

Additionally, while it might be possible to avoid a soils report on simpler, flat lots — by deepening footings until hitting competent soil — there are critical exceptions. If your lot is sloped, or if the ADU footprint intersects a slope, a soils report is essential. This is due to lateral forces exerted on the slope by the foundation, creating a risk of foundation or slope failure if not properly engineered. In these challenging conditions, skipping a soils report is not an option from a risk management perspective. The soils report clearly dictates foundation requirements and safety measures for construction on unstable ground or slopes.

Required by the Jurisdiction

Whether you need a soils report depends on your local jurisdiction’s requirements – see our full list by city later in this article. Some cities, like San Diego, waive the need for a soils report, though more and more are requiring soils reports. Chula Vista, Poway, Oceanside, and Encinitas mandate these reports for detached ADUs. National City requires soils reports for ADUs over 500 square feet. Vista leaves this requirement up to the building official whether to require a soils test to determine the soil’s characteristics at a particular location, though we are seeing it required in nearly all cases. Carlsbad requires soils reports for builds >750SF or two stories, while builds 500-749SF must include 24” footings and a 5” slab to waive a soils report (<500 SF are exempt entirely).

Which Jurisdictions in San Diego County Require Soils Reports?

Local regulations will determine if you need a soils report / geotechnical report. See below our assessment for each city we serve in San Diego.

TABLE: Soils Report Requirements for ADUs by City for San Diego County

Source: SnapADU & local regulations

Can the soils report requirement be waived?

Yes, some jurisdictions allow soils reports to be waived for new detached ADU construction. This is usually on the condition that the foundation meet certain enhanced requirements. While that means the foundation will be more expensive, it is typically still more cost effective than obtaining a soils report and adhering to over-excavation. Plus, you do get the structural benefit of a deeper foundation.

Carlsbad soils report waiver

If you’re building an ADU in Carlsbad, the rules around soils reports depend on the size of the structure:

• Under 500 sq ft: No soils report is required.
• 500 – 749 sq ft: The soils report can be waived by using a prescriptive foundation design. This means 24″ deep footings and a 5″ concrete slab.
• 750 sq ft and above: A soils report is required, no exceptions.

This tiered system allows some flexibility, especially for smaller ADUs. If you’re close to the 750 SF mark, it may be worth considering whether a slightly smaller design could help you skip the soils report process and move faster through permitting.

Chula Vista soils report waiver

For example, Chula Vista typically requires a soils report for new construction. However, the city has a Building Form Request To Waive Soil Investigation Report (4597) to waive this requirement provided that:

• The ADU is being built on natural or cut grade (not on imported or fill dirt).
• A licensed Civil or Geotechnical Engineer certifies this in writing, and the note is included on your plans.
• You follow a prescriptive foundation design, which includes 18″ footings and a 4″ slab for one story designs, or 24″ footings for two story.

You’ll still need to submit a formal waiver request, but if approved, you won’t need to do a full soils investigation. Just keep in mind that the city reserves the right to require a report later if an inspector sees something concerning during construction.

Imperial Beach soils report waiver

Imperial Beach also allows requests to waive geotechnical / soils reports for one story detached ADUs under 750 SF. This requires 18″ deep footings and a 4″ slab. If these conditions are met and detailed clearly on the plans, a waiver may be granted during plan review. However, the city notes that if an inspector sees signs of problematic soil or fill during construction, they can still require a formal geotechnical report at that time.

Vista soils report waiver

Vista allows soils report waivers when built on a simple flat lot. If the ADU is being built on a hilly area or a grading permit is required, a soils report will be required. A soils waiver typically costs $1-2K. Additionally, if the ADU is on a flat pad, a letter from a civil engineer or soil’s engineer is required certifying after inspecting the excavation for the footing that the soils supporting the footing is suitable for the proposed design.

When should you get a soils report during the ADU process?

At SnapADU, we order soils reports during Feasibility immediately after our initial buildable envelope assessment and before full design work begins.

We determine where the ADU will be placed on the site. Once the footprint is known, the soils engineer can investigate the exact building pad. If the report recommends overexcavation, deeper footings, or special drainage measures, those findings may impact design choices. Identifying these conditions early prevents costly redesigns and delays.

How Long Does a Soils Report Take?

Most geotechnical firms can complete the initial investigation and produce the report within 3 to 5 weeks, depending on backlog and required review. Any remedial measures that the soil requires can of course add additional time.

For a typical ADU building pad, overexcavation is a 1-2 day operation, plus one day for field testing and verification. Larger or constrained sites can take longer.

Most of the schedule impact is in planning, not digging. The soils engineer must review and approve the work. Field compaction tests need to pass before foundation forms can be set. If field conditions differ from the report, the engineer may revise recommendations. Because these steps are sequenced tightly with foundation work, we plan them early to avoid mid-build bottlenecks.

Grading Plans

Once you understand what the soil beneath your ADU can support, the next question is how the site itself needs to be shaped to create a stable building pad. That’s where grading plans come in. While soils reports deal with what’s under the surface, grading plans address how the land around your ADU will be modified to support proper drainage, access, and constructability.

What is a grading plan and what information does it provide?

Typically a grading plan will provide you with information about the existing conditions on your parcel and how the site will be altered to accommodate the ADU, including drainage and (in some cases) how vehicle traffic will navigate the site. Equally important is that the grading plan will show the topography of the site. This allows the builder to understand where the guest house would fit best and if there is a need for importing or exporting soil to create a building pad vs. building the structure on a raised foundation.

For sloped lots, topography becomes especially critical. One key consideration is the “7-foot to daylight” rule. In general, if there is less than 7 feet from the bottom of your ADU footing to “daylight” (the point where the slope reaches open air), the footing must be deepened or the ADU must be moved farther from the top of the slope. This ensures the structure is adequately supported and complies with building code requirements. A grading plan helps evaluate these slope conditions and determine whether adjustments are needed before construction begins.

​Do I need a grading plan for my ADU?

In most cases, a grading plan is not necessary to build an accessory dwelling unit. Grading plans have been waived by the state in order to streamline the design, permitting and building of the ADU. However, in some cases, the local governing jurisdiction may require a plan. In other cases, it may simply be advisable to get a grading plan, even if it is not required by your county or city. Again, these plans provide valuable information about your lot and help eliminate any expensive surprises you might encounter during construction.

Under what circumstances should I get a grading plan?

A grading plan would be most advisable when there are challenging site conditions. This may include steep slopes or hillsides, areas of concentrated run off, existing improvements and structures, and a host of other factors. It is wise to invest in a grading plan if your lot is unusual in any way, meaning it is not a standard flat lot.

Also, the thresholds for grading plan requirements will vary based on jurisdiction. As an example, for the County of San Diego grading requirements, “All grading within the County of San Diego must be completed in accordance with approved plans and permits. Work that involves movement of more that 5,000 cubic yards of material must be done under the supervision of a civil engineer.”

Upon our initial assessment we can quickly determine the likelihood of needing a grading plan. If a plan is needed, you can expect spending in the range of $5,000 up to $25,000 if civil engineering is required (grading & wall plans, basic drainage memo, standard SWQMP, erosion control / BMP plan).​

We try to mitigate potential setback and grading issues by considering the overall costs when placing your casita on the site plan, so you will know early on if this is a necessary expenditure. If you have a challenging lot, getting a grading plan is a bit like an insurance policy. ​Read more about actual ADU insurance policies.

Reach out to experts for your unique situation

Overall, these soils and grading reports act as a safeguard to maintain the integrity of your new accessory dwelling unit. Though they are not always required, there are special circumstances where each of these reports are necessary to ensure a safe build that will last a lifetime. You will be made known early on if your lot fits into either of these circumstances, to eliminate any unknowns regarding your ADU. Check out our additional information if you would like to learn more about your city and its requirements.

The information presented in this technical blog is based on our extensive expertise in the field, drawing from our hands-on experience in design, development, and implementation. While we strive to provide accurate and up-to-date information, it’s essential to note that the rapidly evolving nature of the ADU space may result in changes that could impact the details discussed here.

At SnapADU, our goal is to equip our readers with valuable insights and practical knowledge to navigate the ever-changing accessory dwelling unit landscape. We encourage you to reach out to our experts for personalized guidance and to stay updated on the latest developments in the field.