Building construction has significant negative impacts on the environment. However, various measures can mitigate these impacts, including using wood as a building material and improving the building and construction process. The need to renovate and extend existing buildings increases with urbanization and a growing population. Wood is an attractive option for adding new floors to existing structures because of its superior strength-to-weight ratio, allowing for the potential to "build on top" of existing buildings. In order to achieve higher levels of sustainability in wood extension building projects, this study aimed to highlight the benefits of adopting a project-based business model approach by studying the technical, economic, social, and environmental attributes associated with timber building extension projects. Unlike the common firm-level business model approach, a project-based business model allows for a comprehensive view of the value creation, delivery, proposition, and capture of all the key actors involved in a construction project. The study results emphasize the success factors of an extension building project and conclude with critical factors related to the business model's resources, activities, and actors, which would enhance the outcomes of a wood extension project. Such factors relate to knowledge and experience derived from wood construction, and to a holistic perspective on planning, and involvement of all relevant actors from the design phase to project completion.