The purpose of the research is to investigate how a model-based design approach can facilitate the decision-making process in the early design phase. The construction client is in a key position to affect the outcome of a construction project by proper decision making during the design. Different design options can rapidly be analysed to assist the client in making informed decisions in the early design phase. The research approach is based on a theoretical framework for a model based design, decision-making methods and the case of a property in Finland in the early design phase with focus on energy performance. The result of this study is to provide guidelines on how a model-based design process in the early design phase can help decision-makers influence the energy performance of a building. The framework is believed to be generally applicable for decision-making in the design process.