An ADU created by converting an unfinished basement into a permitted, code-compliant living unit.
Most common in older Bay Area and LA homes (pre-1960s) where full basements with 7'6"+ ceilings exist. Rare elsewhere in California where slab-on-grade construction dominates. Major cost drivers: underpinning to lower floors ($40k-$120k), waterproofing ($20k-$60k), egress windows + window wells ($4k-$10k each).
Part of the BackyardADU California ADU Glossary — plain-English definitions of the regulatory, construction, and financing terms you'll encounter during a California ADU project.