An ADU built as an addition to the existing main house, sharing one wall.
Typically 5-15% cheaper than a detached ADU of the same size because you skip a full foundation and one wall of framing. Trade-off: sound transmission and shared structure can reduce rental privacy. Subject to the same state-law size and setback rules as detached ADUs.
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.