This research has investigated the urban policies about mass housing at the beginning of 20th century. This was a unifying and crucial issue of several modern experiences to which many European cities developed adequate answers showing a wide range of distinguishing design approaches and solutions in terms of city planning and architecture. Given the concrete qualities and significant quantity of housings built, this research assumes Vienna and Frankfurt as the more convincing results: Hof and Siedlung constitute two alterative models, each of them refer to a specific way of envisaging the city. The principal goal of this research is the production of new knowledge about these two experiences. This has been achieved through re-drawings, which were elaborated starting from original archival items, and the comparison between them searching the highest degree of homogeneity. The main analytical criteria are: the density, the relationship between typology and urban morphology and the arrangement of outdoor spaces. Density occupies a central role in the nowadays debate, then calculating and comparing the data about the case studies will be useful, mainly thanks to objective documents, in order to direct design choices of the today urban development. The second analytical criterion has provided the documentary basis necessary for a thorough knowledge of Höfe and Siedlungen examples. In fact, referring to the current and specialised literature it is quite rare to figure out the spatial principles and the way of assembling as well as to draw their comparisons. The issue of outdoor spaces and their layout is equally fundamental and very few explored. It takes on a particular interest with regard to nowadays debate over the role and function of nature in the city. As a consequence, the research does not stick at investigating these experiences from a historical point of view, but it aims to consider the case studies analysed as urban organisms which fully-fledged belonged to the contemporary city. Vienna and Frankfurt are, hence, opposite phenomena, but complementary, as much for approaches as for results, from an architectural and planning point of view. In line with these premises, this research recognises Vienna and Frankfurt as "extreme polarities" by the means of them it is possible to go thoroughly into the understanding of the mass housings which played a crucial role in the European history. The dissertation proposes a brand-new interpretation and apt tools for architecture field through comparing the outputs of the two models in a homogeneous way and beyond any preconceptions. Ideally, the method employed in this research will be applied to other urban realities so as to broaden the knowledge and to return the complexity and inventiveness of architectural experiences designed in the inter-war period which so far have been investigated in a fragmentary way.