Sustainable retrofit can increase a building's performance and extend its lifetime. Common sustainable retrofit guides usually ignore the building's indoor environmental quality (IEQ) and occupant satisfaction while only focus on the energy efficiency. During the retrofit, prioritizing improvements to IEQ parameters that are critical for increasing occupant satisfaction but do not considerably increase energy consumption is crucial given the budgetary constraints. This project aims to identify different building experts' perspectives on the impact of IEQ factors and underlying parameters that contribute to occupant satisfaction and energy efficiency in office buildings, and develop an index that ranks the IEQ parameters' overall cost-effectiveness in a sustainable retrofit. The study surveys the views of 30 carefully selected building experts (ten architects, ten building engineers, and ten building assessors) in Australia using multiple criteria decision-making method. Results show that the building engineers and assessors regard thermal comfort as the most critical factor influencing occupant satisfaction, 54.2% and 57.6%, respectively. Alternatively, the architects regard visual comfort as the most important (38.8%). All expert groups have a similar opinion on the relative importance of IEQ factors in office energy consumption. An IEQ Effectiveness index is developed to evaluate the cost-effectiveness of improving a specific IEQ parameter in achieving high occupant satisfaction while not demanding extra energy input. Visual and acoustic IEQ parameters turn out to be ideal IEQ parameters that can be prioritized in a retrofit. Results from this study can provide guidance for the decision-making process of the office building sustainable retrofit.