As a measurement, built density (coefficient of land surface to be built and total floor space) has been the object of numerous researches on urban morphology and architectural forms during the twentieth century. However, for the last ten years, its sensory and qualitative dimension seems to be particularly valued: the perceived density. In multi-unit housing, dense, innovating and compact forms show a significant change of architectural and urban conception. In fact, project practices seem to have been marked by three major changes. Firstly, the importance of the precept of sustainable development, attempting at reducing urban sprawl. Secondly, the increased interest in urban values and the idea of "building a city within a city". Lastly, the current asserted need to redefine density: on one hand, new means are being researched to control it, on the other hand there is a widening of its definition due to the main impulse of architects and researchers. Within this framework, it is illuminating to reconsider the multiple disciplines using this term, as well as the distinct spatial scales tackled in this notion and changes in its use through time. However, focusing only on built density, the present research concentrates on the architectural and urban framework, also known as physical density. It is measured by a density figure but is also qualified by the perception of density. We propose as preliminary approach, an examination of this perceived density in fields where it is more frequently used, such as literature, painting and music. Another important notion of this research requires definition: architectural and urban innovation. Standing out from pure invention, this type of innovation consists essentially in a reinterpretation of previous architectural and urban productions worth naming "retro-innovation". Despite a housing program dependent on past models reluctant to rapid and important changes, radical innovations do occur, though more seldomly. Several ''moments'' of the collective housing architectural history allow for an updating of the different stakes manifested in the density – innovation relationship. Density perception plays a large role in a particular problem emerging from these two notions in contemporary Switzerland. The issue is how encouraging people coming back to the urban centers by proposing larger apartments with more square meters and urban architecture composed of numerous and various spaces, but protecting them at the same time from too close a proximity between neighboring housing and from environmental nuisances. The state of the art brings to light the enormous influence of the researcher and anthropologist Amos Rapoport on perceived density starting with his article, "Towards a Redefinition of Density", published in 1975. For more than thirty years now, architects, psychologists, city planners and geographers refer to this work when defining this notion in their respective fields. As a consequence, it nourishes several reflections in this research. Following these various observations, we propose the analysis of fourteen contemporary multi-unit buildings. They represent the current urban stakes at play and show the architectural and urban conception facing the double requirement of density: to insure a high enough floor area ratio (measured density) and to qualify the housing density (perceived density). In parallel, this research determines if this conception, unique to dense multi-unit housing, tends to modify architectural practice. In the first phase, we examine seven rectangular housing units, in which the width of the building is significant, and seven courtyard buildings in order to examine the exterior spaces contained by the building's volumes. In the second phase, we focus our attention on the wall expression of the fourteen buildings. In this way, these distinct analyses offer three new definitions of the perceived density expressing conception aspects particularly significant for understanding the architecture and urbanity of these dense multi-unit buildings. As a fundamental knowledge, it is shown that density, within the corpus, orientates the architects project's mode. The frequent reinterpretation of paradigmatic dense lodgings is one of the most striking proof of this conceptual change searching for innovations in the register of collective housing. So, in the density of depth, introversion and anonymity on the exterior seems to encourage distinct individualized space within the building as common domestic spheres only opened towards the inside of the apartment. In several cases, the usual urban and public spaces are integrated into the private interior of the apartments as if to domesticate a certain urbanity. In the density of the void, the proposed introversion of semi-public courtyards seems to help maintaining a larger sense of community in these multi-unit buildings. Altogether, it demonstrates a particular capacity to produce small cities within the city, at the same time independent and linked to the city. Finally, through the analysis of wall density, the exterior materiality and expression, dynamic or static, monumental at times, present a decidedly urban architecture enhancing a compact city in quest of new inhabitants.