For several years, the so-called weak culture has been gradually replacing the strong culture that had been typical of the second half of XX century. The economical crisis, along with the climatic changes, has caused a slow, though ineluctable, change of course. Hence, recycling, saving, re-habiting, regenerating, reusing become the keywords of design, which according to differing scales faces the issues of the use of land, of landscape as common good, of built territories where community space is supposed to be found, of buildings that need to be fit. For a long time laws, regulations, guidelines, agreements and programmes have been focusing onto the concepts of economical, political, technical and environmental sustainability. Since the capital of knowledge, history and culture is an inalienable value, to which any creative and interpretative thought must be addressed, the task we are called to cope with is thinking in terms of sustainability. The task of design is often to prefigure scenarios and visions, where architectural thought faces the need for sustainability, reversibility, slightest action self-building and the unfinished. Well, why this title? Because retrofitting is carried out on existing buildings in differing ways according to the peculiar relationships with the building itself. Sometimes an effort is made in order to make the intervention invisible, some other times retrofitting is a chance to redefine the meaning of a building both by adding elements or parts and/or restoring an original state; some other times retrofitting becomes the tool for a complete and upsetting alteration that make the original building no longer recognizable, hence it is the building itself that becomes invisible. With regard to this array of possibilities, the most appropriate cases, i.e. the ones that catch retrofitting for energy efficiency as a good chance for architecture, are the first ones, when the original building requires a ‘silent’ upgrading, or the second ones, which comprehend both the possibilities of adding and restoring in favour of architecture. Among the retrofit case studies, which are examined in this research programme, we are focusing on those that fulfil these principles rather than on those that induce a state of displacement. Thus, three designs for single-family houses, two in Italy and one in Spain, deal with the issue of redefining the meaning of architecture while working on the house typology, on its relationship with the urban context and landscape, on the refurbishment of interiors and on the relationship between interiors and outdoor space. The intervention on a second half of XX century social housing block in Porto restores the original look to a building, while assuming its typical XX century formal features as a design theme. The interventions carried out on two massive public building for education, an Elementary School in Cepina (Valdisotto, Italy) and the Department of Civil Engineering and Architecture in Catania, make an outstanding redesign of the façades, probably aiming at increasing their visibility and recognisability as social architecture. Finally, in the project of the City of the Other Economy, the issue of energy efficiency improvement is developed with regard to a wider regeneration and reuse project of part of the former slaughterhouse in Rome. Pieces and parts of a former industrial complex are linked through new elements, which are entrusted with many roles.