A mixed-use building is an enclosed structure at least 10,000 square meters in gross floor area with 2 or more primary usages, and where at least 2 of these primary usages fall into different top-level categories of usage. For example, a building combining residential condominiums with rental housing would not be mixed-use, but a building combining commercial office space with a hotel would be.

The most common combinations of mixed usages involve office, residences, lodging, and retail. The presence of parking in any quantity does not make a building mixed-use, even though it constitutes a distinct top-level usage category. Retail is normally not a determining factor in a mixed-use building except when the retail component is very large, such as a shopping center, department store, or hypermarket.

Small buildings (under 10,000 square meters) are not normally called "mixed-use buildings", though they quite frequently combine shops or dining with small offices or a few housing units.