The EU building sector is a main contributor to greenhouse gas emissions, which need to be cut as part of the global response to anthropogenic climate change. This cut can be realised through improvements in building energy performance, such as optimisation of facade design. The early stage of the architectural process has been identified as the ideal time to implement such sustainable design choices. There is need for simple guidelines and tools to provide quantitative data to support these architectural decisions. BIM and parametric design can provide this, by facilitating model-based analysis and simulation, as part of an unbroken flow of information through the design process. This study uses Dynamo (the visual programming add-in for Autodesk Revit) together with the Honeybee and Ladybug environmental plugins, to conduct daylight, solar heat gains and thermal losses analyses and simulations. The aim was to identify limitations and opportunities in using Dynamo-Revit, to establish an optimal range for glazing-to-wall ratio (GWR) and to provide some simple room-sizing guidelines for architects in the early stage of the design process. The Dynamo-Revit workflow was found to be effective for specific projects, but difficult to perfect for multiple different projects. An optimal range for GWR was found as 30-40% for east, south and west-facing rooms and around 50% for rooms facing only north. Results were tabulated, linking room orientation and depth with estimated daylight access, solar heat gains and thermal losses. The results were reasonable, but could be improved by the use of more sophisticated analysis and simulation techniques, which should be facilitated by forthcoming development of Honeybee and Ladybug in Dynamo.