An outbuilding, sometimes called an accessory building or a dependency, is a building that is part of a residential or agricultural complex but detached from the main sleeping and eating areas. Outbuildings are generally used for some practical purpose, rather than decoration or purely for leisure. This article is limited to buildings that would typically serve one property, separate from community-scale structures such as gristmills, water towers, fire towers, or parish granaries. Outbuildings are typically detached from the main structure, so places like wine cellars, root cellars and cheese caves may or may not be termed outbuildings depending on their placement. A buttery, on the other hand, is never an outbuilding because by definition is it is integrated into the main structure.

Slave quarters in the United States, sometimes called slave cabins, were a form of residential vernacular architecture constructed during the era of slavery in the United States. These outbuildings were the homes of the enslaved people attached to an American plantation, farm, or city property. Some former slave quarters were continuously occupied and used as personal residences until as late as the 1960s.