A Junior ADU is a unit no larger than 500 sqft created within the existing footprint of a single-family home — usually a converted bedroom + small kitchenette, with a separate exterior entrance.
JADUs are the cheapest ADU type ($60k-$120k typical) and faster to permit since no new exterior structure is built. Trade-off: a JADU that shares sanitation facilities with the existing structure requires the owner to live on the property, in the house or the JADU; owner-occupancy is not required if the JADU has separate sanitation facilities, or if the owner is another governmental agency, land trust, or housing organization. Efficiency kitchen (no full oven required) is permitted.
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.