This research is about strategies of urban actions on public or open spaces, that aim to provide currency and vitality to the city of Santa Coloma de Gramanet that is a multicultural suburb of a contemporary city like Barcelona, with the support of the participation of citizens together with artists, initiatives, grassroots etc. Today it is a clear fact that different methods of design processes for the city as well as distinct understandings of the urban planning are needed for creating/designing lively contemporary cities that are growing with their variety of possibilities. The research contains two parts. The first part tries to explain how the idea of urban design has been changing after it was realized that methods of modernism do not provide the needs of the cities anymore. After the 2nd World War in Europe, understanding of freedom, importance of the perception of an individual, the concept of participation as well as social, cultural and economical changes and their reflections on the urban planning criteria have led the cities to be more free and homogenous places. These impacts have provided the foundation for the necessity of new strategies for urban planning. Rapidly grown cities after the industrialization period in modern era are dealing with the problems of unused, leftover spaces. On the other hand, the potential of these spaces for the planners, architects, artists, and citizens cannot be ignored. By public-centered town planning, public open spaces or urban spaces, can improve the city not only in a physical way but also in economical and social ways. Even though there is not a certain description of a sustainable city, this research will try to explain how non-centralized, temporal and self-management projects can help the cities to be more vivid and sustainable. The example of Santa Coloma, as a suburb of Barcelona, is also affected by the industrialization. Due to urgent necessities, it has grown in an informal way. (Informal does not refer to illegal planning.) Therefore the suburb has a polycentric form. The second part of this research project explains three different hypotheses for three different parts (Besos Riverside, Can Zam Park and Fondo, as different centers) of Santa Coloma. Depending on the necessities of the place new strategies are proposed. These strategies are considered as answers to the requirements of the spaces and catalysts, which will make use of the spaces that, are already available in the area. As consequences, the strategies, which are proposed for aforementioned areas are trying to create a link for the social and physical context in different parts of the suburb. Results of informality in the city become the new answers for the new strategies.