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: state law permits cities to require owner-occupancy on JADUs (the only ADU type with this allowance). Bathroom can be shared with the main house. 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.