CBC 1809 requires a geotechnical investigation by a licensed engineer for all new buildings on sites with: slope more than 15%, expansive or compressible soils (most Marin), seismic site class D, E, or F (most Marin), or any documented landslide / liquefaction hazard. Report must address bearing capacity, settlement, seismic site coefficients, and (where applicable) slope stability. Marin County requires geo review before foundation permits.