An Accessory Dwelling Unit (ADU) is a small, self-contained home located on the same property as a single-family house. You might know it by another name, like an in-law apartment, granny flat, basement apartment, or backyard cottage, but “ADU” is the most common term used across the U.S.

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ADUs come in many shapes and sizes. Some are created by converting a basement or garage, while others are attached additions or standalone units in the backyard. What makes it an ADU isn’t the way it looks, it’s that it functions as an independent place to live, with its own kitchen, bathroom, and entrance.

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If you're just getting started, it's easy to imagine an ADU as a one-size-fits-all structure, but they’re actually very flexible. That’s why it helps to look at the different types of ADUs and how they fit into your existing home and lot.