If you're planning to build an Accessory Dwelling Unit (ADU) in California, there's a good chance your city or county will ask for a geotechnical investigation report before they'll approve your building permit. It's easy to see this as just another line item on an already long checklist, but a geotechnical report isn't paperwork for paperwork's sake. It's the study that tells your engineer, your contractor, and your building department exactly what's underneath your backyard