In the last years I was working with a lot of organizations, trying to explore how to better integrate advanced building performance simulation into the design and engineering processes for buildings. The struggle often is to figure out in what detail simulations are helpful during different stages of design. I have been working with companies that targeted very early decision making to support real estate developers all the way to companies that provide sophisticated consultancy in very detailed design phases. For me results are not conclusive and I really would like to do much more detailed and structured research. The farthest we are coming with our insights is in the area of supporting the renovation of buildings in two large EU funded research projects (P2Endure and BIM-Speed). Here we suggest that detailed building performance models of the existing buildings need to serve as a first step in the design process. These behavioral digital twin can then form a baseline to explore different building renovation options. A key within these efforts is to generate a baseline of the building behavior that normalized factors that are out of the control of the design, such as, weather or occupancy behavior, that cannot be statistically modeled to allow for fair comparision. From the technology development aspect at our firm Contecht we probably came furthest in setting up parametric modeling tools that allow for early simulations and host these tools through dedicated APIs that we developed in web-based design tools.