ADU stands for Accessory Dwelling Unit. Previously known as granny flats or in-law units, ADUs are dwellings either attached or detached from the main house, that exist on the same lot as the main house. Some examples of other types of ADUs include tiny homes, basement units, backyard cottages, and over garage apartments. They are legal additions to your existing property to increase the value of your current property, increase rentable space, and is a no-brainer for the modern-day investor.