B-28 When is a Geotechnical Investigation Required?

The purpose of this technical bulletin is to clarify when a geotechnical investigation is required as part of a building permit application. The requirements below are intended as general policy guidelines.

Section 1803 of the California Building Code (CBC) outlines specific instances of when a geotechnical investigation and report is required based on soil conditions and construction methods. Section 1803.5.11 requires geotechnical investigations for all structures assigned to Seismic Design Category C, D, E, or F. Sonoma County is in both Seismic Design Categories D and E. Where a geotechnical investigation is required, the content of the report shall conform to the requirements of Section 1803 and the specific requirements of Sections 1803.5.11 and 1803.5.12 for structures assigned to seismic design category D and E.

Version: 02/07/2024
Effective Date: 02/2008
Revised: 05/05/2015, 12/27/2018, 04/01/2020, 10/12/2023

This technical bulletin is for projects submitted on January 1, 2020 or after.
For projects submitted December 31, 2019 or earlier, please contact Building Plan Check.

In all cases, if a geotechnical investigation has been previously performed for the parcel, this report must be included in the application. If the existing report is more than two years old, an updated investigation with current design criteria and recommendations shall be provided. A geotechnical plan review letter from the Geotechnical Engineer must be included with the application. The recommendations of the report and review letter must be incorporated into the project plans.

In situations where existing information is not sufficient for Permit Sonoma staff to waive the requirements of a geotechnical report, a waiver of the geotechnical Investigation requirements can be requested by the registered engineer in responsible charge of preparing the structural plans for the project by using BPC-070 Geotechnical Report Waiver Request (Email Building Plan Check for document: PermitSonoma-PlanCheck@sonoma-county.org), if they can verify that no geotechnical hazards are present.

A geotechnical investigation is required as follows:

1. New, replacement, or accessory structures for Occupancy Groups A, B, E, F, H, I, M, R-1, R-2, R-4, and S occupancies unless specified below – geotechnical investigation required.
2. New or replacement structures for Occupancy Group R-3: single family dwellings, accessory dwelling units, guest houses, pool houses or miscellaneous residential occupancies such as home office, gym, yoga room, art studio, recreational or media room, etc. – geotechnical investigation required when any of the following conditions exist or may exist, as determined by site review or mapped information:
   1. Area of suspected expansive soil
   2. Area of suspected slides, slumps, or soil creep
   3. Area of previous or proposed fill placement
   4. Area without sufficient setback to slopes, steams, or adjacent foundations
   5. Area subject to possible liquefaction
   6. Area of suspected soft, compressible, or organic soil with low bearing capacity
   7. Area of high moisture content in soil
   8. Area subject to high erosion
   9. Area of soft soil due to past deep ripping or cultivation below minimum foundation depth
   10. Area within 1000 feet of a solid waste disposal site
   11. Design values in structural calculations exceed presumptive load bearing values per Table 1806.2
   12. Project includes deep foundations or shallow footings bearing on compacted fill material more than 12 inches in depth
   13. There is evidence of structural failure of the existing foundation due to soil conditions
   14. Replacement structures that increase loading to existing foundation
3. Specified new or replacement Occupancy Group U structures accessory to an Occupancy Group R-3: private garages, barns, storage buildings, workshops, and water tanks exceeding 5,000 gallons or other Occupancy Group U’s designated by the building official – geotechnical investigation required when any of the following conditions exist, as determined by site review or mapped information:
   1. Area without sufficient slope setback
   2. Area without recommended setback from stream
   3. Proposed buildings or excavations are potentially hazardous to other structures
   4. There is evidence of structural failure of the existing foundation due to soil conditions
   5. Area of previous or proposed fill placement
   6. Replacement structures that increase loading to existing foundation
   7. Project includes deep foundations or shallow footings bearing on compacted fill material more than 12 inches in depth
4. Foundation upgrades – geotechnical investigation required if:
   1. There is evidence of structural failure of the existing foundation due to soil conditions
   2. Area without sufficient slope or stream setback
   3. Project includes deep foundations or shallow footings bearing on compacted fill material more than 12 inches in depth
5. Flood elevation projects to raise structure above the base flood elevation – geotechnical investigation is not required if the existing foundation is found sufficient. If the existing foundation is replaced or retrofitted, a geotechnical investigation may be required per the other sections of this bulletin.
6. Remodels with no increase in square footage and no foundation repair or upgrade – geotechnical investigation not required.
7. Swimming pools – geotechnical investigation not required if the design complies with all of the following:
   1. Expansive soils, if present, are assumed in the pool design
   2. The pool is not in any area listed in Section C, Items 2 through 12.
   3. Design values in structural calculations do not exceed presumptive load bearing values per Table 1806.2
   4. If a geotechnical investigation is not required by the product listing
8. A change of occupancy will be treated as a “new” structure for the proposed occupancy.  See sections A, B, and C above.
9. Retaining/Basement/Foundation Walls with more than 6 feet of backfill – A geotechnical engineer shall determine the dynamic seismic lateral earth pressure due to design earthquake ground motions (CBC 1803.5.12) with specifications for back fill placement, compaction, and material recommendations. Other report requirements listed in CBC 1803 are not required unless the proposed retaining wall design is subject to any of the items listed in Section C, 2-12.
10. Residential additions – geotechnical investigation not required if:
    1. The proposed foundation is the same type as the existing foundation, and
    2. The existing foundation is a perimeter spread footing or slab on grade and
    3. The existing foundation complied with the code when it was constructed, if a building code was adopted at that time, and
    4. The addition has sufficient slope and stream setback, and
    5. There is no evidence of structural failure of the existing foundation due to soil conditions, and
    6. The proposed foundation complies with current code, and
    7. Project does not include deep foundations or shallow footings bearing on compacted fill material more than 12 inches in depth
    8. Where evidence does not indicate variations in rock strata or supporting soil characteristics at the proposed addition area from the area of the existing structure