When Do You Need a Soils Report?

Your ADU's foundation is designed for two things: the building above it and the ground below it. The building is known; the ground sometimes isn't. A soils report (geotechnical report) is how the ground gets known.

What it is. A geotechnical engineer samples your soil (usually small borings in the yard), tests it, and reports its bearing capacity, expansiveness, and stability — with specific foundation recommendations your structural engineer must follow. Cost: typically $2.5K–$6K+ for residential work.

When it's required or wise:

• Hillside and sloped lots — almost always required; slope stability and foundation design depend on it (Module 2.5's warning becomes engineering here)
• Known expansive clay areas — clay that swells wet and shrinks dry cracks slabs built without countermeasures; much of Southern California qualifies, and cities often know exactly which neighborhoods
• Suspected fill — soil that was dumped or graded historically (common near old creek beds, on terraced lots, or where "the yard was leveled at some point") can't be assumed to bear weight
• When the city says so — many jurisdictions trigger a report by slope percentage, map zones, or simply policy; your designer finds out early
• Signs of trouble — existing cracks in the house's slab or walls, doors that stopped closing, leaning retaining walls

When it's typically skipped: flat lots in established areas with well-behaved soil and no city trigger — the plan checker accepts standard foundation assumptions. This is the majority of simple projects.

What it changes if the news is bad: deeper footings, post-tensioned or mat slabs, drilled piers on hillsides, drainage requirements — in short, money (recall Module 6's hillside numbers). But learn the lesson of this whole unit: bad ground discovered by a $4K report during design is a line item; bad ground discovered by a cracking foundation after move-in is a catastrophe. Reports are cheap. Surprises are not.