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<br /> <br />Foundation Requirements for <br />Residential Projects and Accessory Structures <br />(Approved September 22, 2021) <br /> <br /> <br />BACKGROUND: <br />1. Section R401.4 of 2019 California Residential Code (CRC) requires a soils test to determine the soil’s <br />characteristics if expansive soils are likely to be present. <br />2. Section 1803.5.11 of 2019 California Building Code (CBC) requires a “geotechnical investigation” for <br />any structure determined to be in Seismic Design Category C, D, E , or F in accordance with Section <br />1613. <br />3. Exception to Section 1803.2 states, “The building official shall be permitted to waive the requirement <br />for a geotechnical investigation where satisfactory data from adjacent areas is available that <br />demonstrates an investigation is not necessary for any of the conditions in Sections 1803.5.1 through <br />1803.5.6, and Sections 1803.5.10 and 1803.5.11.” <br /> <br />POLICY: <br />Since the majority of jurisdictions in Orange County is located in Seismic Design Category D and due to the <br />presence of expansive soil in some areas, this policy is developed to comply with the 2019 CRC and CBC. <br />Geotechnical investigation report is required for all projects except as prescribed below . <br /> <br />Exception: <br />1. One story room addition(s) to a single-family dwelling/accessory dwelling unit (SFD/ADU) up to <br />1,200 sf of individual areas located in non-liquefaction area. <br />2. One story detached SFD/ADU, or accessory structure up to 1,200 sf located in non-liquefaction <br />area. <br />3. One story room addition(s) to a SFD/ADU up to 500 sf of individual areas located in liquefaction <br />area. <br />4. (a) Room addition(s) to a SFD/ADU larger than 500 sf of individual areas but not more than 1,200 <br />sf located in liquefaction area shall require 4’ of overexcavation. <br />(b) Detached accessory structure or SFD/ADU larger than 500 sf but not more than 1,200 sf located <br />in liquefaction area shall require 4’ overexcavation. <br />5. One story detached accessory structure up to 500 sf located in liquefaction area. <br />6. Second story addition above existing first floor within the footprint of existing dwelling. <br />For item 6, a letter from a California Licensed Geotechnical Engineer or Civil Engineer practicing <br />soil engineering is required, stating the condition of existing foundation and supporting subgrade <br />soils and providing foundation recommendations due to the added vertical and lateral loads from <br />the proposed second story addition. <br /> <br /> <br />Page 24 of 290