At the turn of the 20th century, Vienna was the capital of the Austro-Hungarian monarchy. The Empire spanned over many different countries and Vienna was therefore considered as the center of Europe. During the industrialization, the population of Vienna increased up to 2 million within a few decades. Workers from all over the monarchy moved to Vienna into the working-class quarters outside of the Viennese belt. Vienna had to deal with the rise in population and its concomitant demand for new housings. Back then, the working-class districts were built, which shaped the character of the city for decades. This heritage from the past is still visible nowadays. Beginning of 2014, Vienna is in a similar situation as back then. The current population of 1.7 million is expected to rise up to 2 million again. What will be our heritage for the future? The city must adapt to the challenges. Two approaches are currently heavily discussed. The first approach would be to expand Vienna into suburban areas containing satellite towns or large-scaled housing quarters. The second would be to recompact the city center. Both approaches have their advantages and disadvantages. A city expansion correlates with an enormous loss of base areas and the need of building new infrastructures like roads, public transportations and new shopping possibilities. This is in strong contrast to recompaction which involve general refurbishment, roof extensions and refilling of gaps between buildings. Thereby the population density is increasing without a decrease in the quality of life. Furthermore, the general infrastructure already exists. In combination with the close proximity of everyday's needs the energy efficiency is much better and the value of the city is increased. This diploma thesis deals with that topic of recompaction of Vienna. It is divided into two main parts, a theoretical and a practical one. The theoretical part describes the Viennese period of promoterism and its challenges as well as the current situation in Vienna. Furthermore the two models for the future development, city expansion and city recompaction, will be discussed in more detail. The theoretical part ends with important laws for roof expansion in Vienna and the complex problems resulting from it. The practical part deals with the design of a roof extension of a building constructed during the period of promoterism. It shows the architectural realization of that task. The aim of that design is to show how problems like energy efficiency and sustainability can be primarily solved by planning methods. The focus is on the architectural planning. The choice of materials and their correct usage should mainly solve the problems. That design demonstrates the connection between the old and the new and shows how to use the strengths of one's part to balance the weaknesses of the other part.