 What exactly does that stand for? It's not a very sexy term, is it? No, no. I always lament that there isn't a better word for it, but 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.