The study presents how numerical simulations can be used and complexity to define key indicators for microclimate in eco-district modelling. For eco-district design, policy makers require decision support tools for integration of local environment issues with building design. In a first approach we analyse main environmental parameters that impact building energy performances and open-space thermal comfort. This is a key issue considering the local urban heat island impacts on energy and health. The urban area Cité Descartes is located in Paris suburbs (France) and has been selected in the French eco-district program. One objective is to renew the surroundings of a local train station that separates a low-income residential area and a business district. The study focuses on the central block of buildings covering a total area of 120,200 m 2. The objective of the study is to evaluate main micrometeorological parameters influencing thermal comfort and building energy efficiency in the selected urban scenes; e.g. the wind flow patterns highlight discomfort zones for winter and summer periods. Finally, numerical simulations tools are used to underline the spatial and temporal variability of urban states which makes both difficult the analysis of one urban design and the decision process between several urban cooling strategies. The perspectives to develop new decision support indicators for energy and environmental performance are reviewed and proposed to develop both necessary simulation tools and decision support systems.