An Accessory Dwelling Unit, or ADU, is an additional living space on your property, most commonly in your backyard. ADUs are popular home additions because they can be optimized to fit your specific needs and their often-detached nature means you can expand your living space without the hassle of home construction or remodeling. Whether you call it a granny flat, cottage, casita, or in-law suite, an ADU comes in many sizes and styles.