Future energy systems will require buildings to be able to manage their energy demand and generation in dynamic ways. The technical realization of such energy flexibility in buildings in response to local climate conditions, occupant needs and grid requirements is currently quantified in research and development projects, many of which take place in newly erected buildings as flagships for the related digitalization and seamless automation of technical building services. However, building stock and facility management portfolios tend to consist of existing buildings with differing, highly diverse performance qualities that may pose a problem for generic solutions. The objective of this paper is to highlight potential options, and to sketch an engineering perspective of making existing buildings energy-flexible. For this purpose, on-going work in various control research projects is selected to present issues in (1) the development of a suitable controller, (2) home and building automation design, and (3) building commissioning and diagnostics for future building controls. Non-technical requirements for quality of performance in these cases are summarized. In conclusion, and based on the reviewed projects, a potential strategy for building management is the avoidance of risks and costs associated with the introduction of energy flexibility by using published standards and open protocols for automation, and by documenting their as-operated status in a digital format in all buildings.