Viennas cityscape was characterised in the time of the foundation period. No other stylistic era is as heavily represented in Vienna as the period between 1850 and 1914. A third of existing structures originate from this time and in the central districts, this figure can even reach 80%. The reason for the large proportion of foundation period structures lies amongst others, in the fact that the construction style that was developed in the foundation period, was not only extremely sturdy, but also extraordinarily sustainable. By adapting the floor plans and implementing improvement measures in the building's thermal performance, foundation period buildings can be brought up to todays standards in a very simple and cost-efficient manner. The city of Vienna supports the renovation of the old buildings and promotes the revitalisation of the city. However, the interests of the city do not lie in the up-keep and renovation of the existing buildings alone. Since 2001, Vienna has seen a strong increase in population, particularly in buildings built in the foundation period - something, which the city of Vienna must respond to. In the first instance, the city has attempted to raise the height of the buildings and developing the roof-levels in order to achieve more living space. In my dissertation, I would like to set myself the challenge of discovering other solutions to increasing the density of population in the city of Vienna. Using an example of a city block from the foundation period, I will demonstrate how a new structure can fit into an existing group of buildings, is able to offer a variety of possible uses and which is able to accommodate the increase in population, which the existing structure cannot. The construction site of this structure will not be at ground-level, since the ground-floor levels in the inner-city districts are already very widely used and densely populated. The site of construction will be multi-level firewalls, located on the inner side of foundation period buildings. Without windows on it these Firewalls look like useless "blind" walls, which lie on the periphery of a property. As a result of the standardised building style of this building extension, the developed structure can be used universally in order to increase available living space in foundation period buildings.