Smart buildings combine ICT and IoT technologies (e.g., smart appliances, sensors, actuators and smart meters) in order to provide supervision functions for Building Management Systems (BMS), which are capable to monitor and control both their physical elements (e.g., energy systems, network and storage facilities) and conceptual elements (e.g., building users, usage scenarios, security and trust and intrusion). To develop such systems, there is a need for a common information base which is expressive and flexible enough to describe building elements, their characteristics and interrelationships, as well as the constraints that apply to them. Despite the plethora of existing BMS models, there is still a lack of a common model showing a large compatibility and interoperability with existing BMS. Therefore, we introduce in this paper FUSE-IT ontology which provides a unified view of smart buildings by merging IoT/BMS ontologies such as Semantic Sensor Network (SSN), Smart Appliances REFerence (SAREF), Smart Energy Aware Systems (SEAS), among others. The obtained model is the basis of smart BMS we are aimed to implement within the FUSE-IT project to ensure global physical and cyber security, trust and safety in critical sites