One of the most important aspects a design team needs to consider fairly early on is that of energy saving, cost, thermal comfort and the effect on the environment in terms of CO2 emissions. This paper presents an online collaborative tool that allows the design team to collaborate - from distance - on providing a general description of a building which can be used to generate sufficient data to drive a valid analysis using a detailed thermal model at the early sketch stage of the design process. This tool - called CASD - guides the user through the input process using a series of screens giving options for keywords used to describe the building; comprehensive default data built into the software are then attached to these keywords. The resulting data file is a building description that is the best possible interpretation of the design intent. This can then be used to assess options and guide towards a final design.