Simulation modelling and Lean principles have both been applied in the construction industry to improve work processes. The outcomes from their implementation are outstanding and have motivated construction researchers to seek means by which other aspects of construction production could benefit from this development. Therefore the aim of this research is to use computer simulation as a tool for assessing the impact of applying Lean principles to design processes in construction consultancy firms to aid in decision making at early stages of construction projects. A comprehensive model for the design process was built before the principles of Lean construction were depicted in the model. Through a detailed case study, it was concluded that applying Lean construction principles to the design process significantly helped to improve process efficiency, in terms of reduced process durations and increased resource utilisation.