Several ontologies exist that model aspects of home or building automation systems for specific use cases. However, no comprehensive approach exists, that models building automation systems in a modular way to be usable in various use cases and tools. The paper proposes the BASont that addresses various use cases over the life cycle of a building automation system from design, to commissioning, to operation, and refurbishment. The use of the BASont is demonstrated for a data retrieval and self-commissioning use case. © 2012 IEEE.