An accessory dwelling unit, usually just called an ADU, is a secondary housing unit on a single-family residential lot. The term “accessory dwelling unit” is an institutional-sounding name, but it’s the most commonly used term across the country to describe this type of housing. While the full name is a mouthful, the shorthand “ADU” is better.

The fact that it’s a secondary housing unit, rather than a given structural form, is what defines an ADU.. But when we’re learning about concepts, it’s natural to want to know what that concept looks like in the flesh. We want to visually embed the design concept in our brains as a tangible object that we can mentally reference. However, ADUs vary in their physical form quite a bit, so allow me to broaden that mental model by exposing you to the range of common ADU types, in order to better understand what they are.