ADU Grading Plans and Soils Reports: When Do You Need One in San Diego?

If you have a challenging sloped lot or unusual soil conditions, you may need to consider a grading plan or a soils report for your granny flat in San Diego. Read on to learn more.

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

Typically a grading plan will show existing conditions 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 ADU 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.

​Do I need a grading plan for my ADU?

In most cases, no. Grading plans have been waived by the state in order to streamline the design, permitting and building of the ADU. In some cases, the local governing jurisdiction may require a plan. Or in some cases, it may be advisable to get a grading plan even if not required.

Under what circumstances should I get a grading plan?

A grading plan would be advisable when there are challenging site conditions. This may include steep slopes or hillsides, areas of concentrated run off, existing improvements and structures, or a host of other factors.

It is the rare case that a grading plan is recommended by SnapADU, as most sites are not so complex as to dictate the need for a grading plan. Upon our initial assessment we can quickly determine the likelihood of needing a grading plan. If a plan is needed, grading plans can cost in the range of $6,000 – $10,000.​  We try to mitigate potential setback and grading issues by considering the overall costs when placing your ADU on the site plan. But if you have a challenging lot, getting a grading plan is a bit like an insurance policy. ​

What is a soils report?

soils report is an analysis of soil conditions and how those would affect a building foundation. The main purpose of a soils report is to flag any dangerous soil conditions and recommend design criteria that could mitigate the concerns.

There are two steps to a soils report. The first step is the initial site investigation (before any grading work), from which the formal report is produced. The second step is on-site testing at the time of digging the foundation. This testing is typically specified in the soils report and has to be performed by the engineer of record to confirm that the actual field conditions match with the report’s recommendations.

Do I need a soils report for my ADU?

Generally, the answer is no. Most jurisdictions waive the need for a soils report, though some are requiring it. Chula Vista mandates these reports for detached ADUs. Carlsbad and San Marcos require 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.

While the soils report is helpful in understanding the underlying condition of the building pad, it is not necessary to know this information prior to building. A typical soils report will cost around $5,000 or more. 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.​​

How does SnapADU balance the need for a soils report against mitigating risks?

We balance this risk with a conservative ADU design. Having a foundation fail can be a big problem. That is why we design our ADUs to meet UBC (Universal Building Code) minimums. This means we assume the worst case soils conditions (e.g. soil with minimal ability to support a foundation) and design the foundation with that in mind. This typically means we use oversized footings (footings are the structural portion of the foundation that directly support the walls). While these larger footings do require a bit more concrete and rebar, the cost for this is far less than having to produce a soils report and conducting field testing during construction to confirm the findings in the soils report.