All demographic forecasts indicate that population density in urban areas will rise considerably in the coming decades. In order to limit urban sprawl and the resulting encroachment into agricultural and natural areas, urban areas need to be densified inwardly. This is also a requirement set out in the latest Spatial Planning Act (SPA), which takes into account a wide and variable range of old urban buildings. In order to accommodate a growing number of inhabitants over a constant ground surface, living spaces must reach towards the sky (because no one really wants to live underground). This baseline situation raises the following questions: 1) What building materials and techniques can be used to expand residential surface area by increasing the number of floors of current building structures without disproportionately affecting existing constructions and surroundings? 2) How to ensure that current and future economic, ecological and comfort constraints of users and citizens are taken into account in this vertical growth? Increasing the number of floors of existing buildings implies very different requirements from those of new constructions, particularly in terms of the production and use of construction elements. In order to avoid disproportionate overload of existing buildings, the installed construction elements must essentially be very light: lightweight materials can be used to produce normal-sized construction elements or high-strength materials can be used to produce very thin elements, decreasing overall weight in this manner. Economic and environmental performance depends on the materials, manufacturing and implementation methods used. Emphasis should therefore be placed on prefabrication or simple on-site implementation. Similarly, performance is also determined by the thermal envelope and finishing work (‘non-load-bearing’) needed to ensure the comfort of users. This, in turn, will mainly depend on the requirements arising from current and future building physics (which will increase as a result of dense cohabitation and climate change). The project should lead to specific technical proposals for the densification of urban living areas through the economic raising of existing buildings. These construction techniques will have to meet the multiple, complex and increased requirements resulting from denser cohabitation and more extreme temperatures associated with climate change. At the same time, a balance with nature should be achieved in terms of preserving natural resources and reducing both greenhouse gas emissions and energy consumption.