To reduce a building's impact on the environment, governments and certification boards encourage the use of innovative and collaborative design processes such as Integrated Design (ID). ID proposes upfront, stakeholder-engagement and collective decision-making to improve life-cycle building performance. Although ID's potential is theoretically well-founded, there is little empirical evidence of its effectiveness. This study seeks to validate the extent to which ID effectively improves project performance in terms of its reduction of environmental impacts. Three Canadian building projects, certified LEED and integrating various environmental strategies, are examined. The research team first identified and evaluated strategies aimed at reducing the buildings’ impacts. Then, it analysed the decision-making process and measured impact reductions comparing reference buildings, schematic designs and construction documents - using Life Cycle Assessment (LCA) tools and Building Energy Simulations (BES). Results show a 60% reduction in global warming potential (GWP) and 62% in energy consumption in the case studies. They also underline five challenges for ID practices: tool complexity and accuracy, missing information, reducing embodied energy in high-performance buildings, poor environmental design decisions, and decision-making based on green certification credits. Opportunities to overcome these challenges include deepening professionals’ knowledge of Life Cycle Assessments and developing more effective energy simulation tools. The findings can help improve ID processes and protocols to reduce a building's impact on the environment.