There is widespread recognition of the importance of the built environment in tackling the current energy and sustainability challenges. It is well known that with existing buildings constituting more than 98% of the building stock, the greatest impact on reducing building energy consumption in the US will result from retrofitting existing buildings. These need to be based on radically improving their energy efficiency and overall performance. The aim of the project, on which this paper is based, is to develop an integrated process model and decision support system (DSS) that can provide proactive guidance to facility owners/managers in undertaking high performance green retrofits of existing buildings. This is being addressed through detailed case studies of a number of high performance green retrofit projects, the modeling of the retrofitting process, the identification of key decision criteria, and the development of an appropriate decision support system (DSS). The DSS will provide guidance on technical alternatives available at each decision point along the retrofitting process, including the link to associated energy performance outcomes and cost implications. The paper presents the approach being adopted in the development of a process model for energy efficient retrofits, and an intelligent DSS that enables informed choices to be made.