Retrofitting existing buildings has the greatest opportunity to reduce the energy consumption in the building market. Even though the opportunity lies in making these design decisions collaboratively, it is difficult to come up with "best project fit" with current techniques by considering all disciplines' priorities. Especially, deep retrofits that provide higher energy targets require integrated design approaches that include the perspectives and priorities of the various disciplines through collaborative decision making within the limits available budget. The aim of this project is to define the critical decisions - when, why, and how these decisions are made in contrast to other design options - by the decision makers in the team. Two deep retrofit case studies will be presented and will deliver integrated process maps and comparative analysis using different delivery methods. The maps and decision criteria are being used in the development of a Decision Support System (DSS) aimed at providing guidance to retrofit project teams by increasing the transparency of the decision making process and highlighting the energy efficiency implications of design alternatives.