An accessory dwelling unit (ADU) is a secondary housing unit located on the same lot as a primary residence, often referred to as a 'granny flat' or 'in-law unit.' These units can take various forms, including converted garages, basements, or new structures built in the backyard, and are designed to provide additional living space while maximizing the use of existing property. They play a key role in addressing housing shortages and promoting sustainable living in California's urban areas.