In recent years, the compact city model has gained prominence as a strategy for sustainable urban development, with urban densification at its core. While densification is recognized as a key planning tool, questions remain about how to use it effectively to achieve broader sustainability goals.

This study explores the role of density through the case of Zurich, a global city known for balancing competitiveness with quality of life. Using Campbell's (2007) triangle of conflicts in sustainable urban development, the research evaluates whether built, population, and functional density contribute to balancing the effects of intensified land-use processes, such as the provision of housing, proximity, and mix of uses.

Focusing on Zurich's Aussersihl district, the study analyzes three planning phases: the industrial city, the modernist functional city, and the sustainable city era. By integrating historical and current GIS data with projections for 2040, the findings reveal a transition from mixed-use residential areas to service-based areas. This shift is driven by modernist planning instruments promoting functional separation and de-densification. These approaches may impede sustainability goals, including functional diversity and density of use. The study calls for updated planning instruments and a redefinition of how density is conceptualized and operationalized in urban planning.

With the revision of the Swiss Spatial Planning Act in 2012 and its implementation 2014, inward settlement development became the main objective of urban design and planning in Switzerland. As Zürich has to cope with a 25% increase in population by 2040, the revision of the city’s building and zoning laws requires a revision of urban densification strategies and the instruments for controlling density and land use. Furthermore, the turn towards inward settlement development opens up a discussion on the social vision of urban design and the ecological and socio-economic consequences of spatial interventions on future quality of life and the inclusiveness of cities.