I investigate the field of tension between the national and the local level in spatial planning from a decision-making perspective. In doing so, I analyse the legal regulation for a large-scale 3G mobile infrastructure development in Sweden with a focus on how participation is expressed both in terms of the ‘law in books’ as well as empirically, ‘in action’. Theoretically, a model of decision making is elaborated, based on two axes: one concerning the decision level on a central or national to local scale, and one concerning what type of knowledge is regarded as the most legitimate in terms of a calculating approach versus a communicative approach. These two issues or approaches to decision making—who decides and based on what knowledge—are of direct importance for understanding the frameworks as well as the practical outcomes of public participation. The case of 3G in Sweden demonstrates how different types of knowledge are perceived as legitimate at different levels in the planning system. For example, appeals against building permits rarely change the outcome of permits issued, and appeals based on fear of electromagnetic radiation are always rejected. The juridification of a given mast conflict meant a development from a deliberative approach, where any concern is heard, to a calculating and expert-based one, where the general stance on a particular topic (such as whether or not the electromagnetic radiation from mobile masts is hazardous) is applied. This means that what knowledge is legitimate depends on where in the permit process it is presented.