Despite covering only 2.7% of the world's total surface area, the world's cities are responsible for 75% of the world's energy consumption and 80% of greenhouse-gas emissions. For this reason, the redesign of cities is essential in the transition towards sustainability. However, planning and designing sustainable neighbourhoods is not a simple task, given that there is no agreement on what the sustainable settlement should be, nor on how this should be achieved. Furthermore, planners have to strive to achieve a balance between financial, environmental, and social goals, and must deal with multiple actors and stakeholders and with site-specific characteristics. The aim of this work is to describe the key determining factors—both opportunities and constraints—found in the process of designing and planning a neighbourhood, based on a case study in the city of Barcelona. In this practical experiment, led by the authors, the ecodesign methodology was applied on an urban scale in the neighbourhood of Vallbona, Barcelona, which occupies an area of 32.6 ha and will host 2120 dwellings. From this neighbourhood ecodesign exercise, it was found that territorial (urban form, urban fabrics, and density; availability of local resources), financial, legal, and political (local government's wishes and leadership) determinants are the most important. It is concluded that there is no single path to achieve urban sustainability, since the design of neighbourhoods in different locations will lead to different results.