The aim of this case study, underpinned by participative action research and design science research methodologies, is to show how design and design management practices can be improved based on a new conception of design activity and lean design management. First, problems related to design and design project management are identified using a triangulation of methods, and a root-cause analysis is conducted. Second, interventions are developed, implemented, and evaluated over two iterations. The methods and practices employed in the organization under study imply it had adopted the transformation view of the conceptualization of design. It was also observed that the organization considered design strictly a technical activity. Both choices appeared to be the root causes of the problems faced by the organization. To complement the transformation view, methods and practices following the flow and value views were introduced. To counteract the strictly "technical understanding of design", "social" concepts were introduced. As a direct result of theory-driven interventions, there were significant improvements in building design processes and design management practices.