The open space in Flanders is subjected to different transformation processes. These processes are related to dynamic interactions within an urbanizing society and to competition for space by an increasing number of functions. Despite the acknowledgement of dynamic land use demands, the current categorizations of land use and land cover are not always able to catch transformations related to e.g. newcomers in land use, ecosystem services and multifunctionality in mixed urban–rural areas. Remote sensing tools and census data are insufficient when studying a complex and intensively used space. This research therefore aims to identify underrated transformations in the study area Flanders (the northern part of Belgium) using two main sources: (i) open expert interviews and (ii) three case studies. The interviews serve to formulate a renewed framework that can be used to describe transformations in the open space. The transformation processes that emerge from the interviews and case studies are compared to those defined and recognized in the current spatial planning policy and in existing monitoring data, in order to identify bottlenecks and options for future spatial planning policy. The major conclusions of this paper are (i) a critical view on the analysis and categorization of functions and open space as it is currently practiced in land use monitoring and land use planning programs, (ii) additional data needs to encompass transformation processes in the open space and (iii) the need for a more integral vision for open space.