You or your life. If it's an office building, it might represent your work life. The dream events that happen in the building can represent events in your life (or work life). If present in the dream, a basement can represent your subconscious, the top floor your spirituality, and the floors in between your current life.
A building can also be a setting, an atmosphere, or provide a tone or mood for certain dream events. For example, a school building might represent a recent learning situation or enlightening conversation.