 ADU, the acronym is short for Accessory Dwelling Unit. We tend to call them backyard houses here. It seems a little more elegant, but if you have a better solution, you're hired. But we define them roughly as a secondary, usually detached, self-contained living space on the same lot as a larger house. Picture a tiny house in someone's backyard and you've kind of got it, though we've seen them in all kinds of shapes and sizes.