Urban densification issues are one of the key challenges for most European cities. In this context, the vertical extension of existing buildings provides a solution to create usable spaces without wasting additional land. Little attention has been given to office buildings, although, given their abundance in the building stock, they offer an important potential. The 'Working Space' project gathers an interdisciplinary research team around the development of an innovative, modular and prefabricated timber construction system. The dimensions of the latter can be adjusted to the structural grid of existing buildings and to a great variety of new typological organizations. The extension’s envelope is based on the principles of bioclimatic architecture and combines active and passive elements. It features a high-performance insulation, provides a smart management of passive solar gains, natural ventilation and free cooling, and offers large surfaces dedicated to photovoltaic energy production and urban biodiversity. The system is composed of local materials with very low embodied energy. This paper presents the project’s outcomes at different scales, ranging from urban design to construction details, and the outputs of an innovative life cycle assessment. A first application of the system, comprising 40 workspaces, is currently being conducted in Lausanne, Switzerland.