The 2021 EPBD recast highlights buildings' central role in the energy transition towards an integrated and renewable energy-based EU energy system. Smarter buildings will support the digitalisation and decarbonisation transition to the benefit of building occupants, owners and the energy system. Improving energy performance and reducing energy demands are even more relevant in the context of the REPowerEU Plan to rapidly reduce dependence on Russian fossil fuels and accelerate the green transition. BuildON seeks to develop a highly replicable, generic solution to deliver intelligent building services, facilitate the integration of heterogeneous systems and technologies, and help Build the next generatiON of Smart buildings. The main objective of BuildON is to develop and demonstrate a Smart Transformer Toolbox at TRL8 to plan and achieve improved energy performance applicable to the broadest range of building typologies and smart readiness levels via a comprehensive demonstration campaign involving five representative real-life use cases. This toolbox will offer affordable, adaptative, close-to-market and easy-to-install SRI-aligned services to continuously Monitor, Assess, Predict and Optimise (MAPO services) a building’s performance. A unified representation of the building, exposed through a Universal Building API, will abstract all field-level communication complexity, allowing the energy devices’ homogeneous real-time monitoring and control. Moreover, the generation of (Maturity Level 3 and 4) Digital Twins will allow simulation and control of a building and its systems. User-friendly decision support and user empowerment tools for facility managers and occupants will be part of the toolbox to explore, understand and expand their buildings' smart capabilities. By providing a unified approach to interacting with buildings, the floor is offered to develop a building app store that significantly accelerates the Smart Transformation of existing and new buildings