The development of an Environmental Site Assessment tool is described which is designed to be used by planners and developers of speculative housing, for the appraisal of whole new housing developments. The aim of the tool is to establish the implications of trade-offs between passive solar designs and more conventional suburban housing, while achieving a particular site density target. The tool can determine what the effect of changing housing density will have upon a passive solar development. The whole life financial and environmental costs are then calculated accordingly for a housing project. Although the research has involved detailed analysis of building performance, the focus of the tool is on the footprint of the house and the early stage design of housing layouts. Thus, it can have an influence on passive solar performance from the earliest part of the planning process, even before the sketch design stage. The intention, with further development of the tool, is to provide a comparative analysis between dominant current approaches and emerging environmental methods of construction in housing developments. At this stage, the tool is a prototype which aims to simplify a focused range of complex planning issues to give an informed and best case scenario of anticipated costs and emissions over the lifecycle of housing developments. Although this tool is aimed at smoothing the adoption of more sustainable housing practice during the planning stage, by providing discourse between local authority planners and the builders of housing developments, ultimately the main aim is to benefit the environment by reducing emissions and the homebuyer by reducing household energy costs and increasing comfort.