What is an ADU? It stands for accessory dwelling unit, and to help you better understand what they are, I’m coming to you from an ADU that’s in the process of being built right now. Thanks to recent changes in zoning laws, California has many options to let you build a second unit on your property. That can be helpful if you’re looking to have family come stay with you, or it can serve as a source of income if you rent out the extra unit.