This paper reports on a case study investigating how civil defence interests may influence land-use planning through legislation on national interests. The project brought together two disparate areas and involved collaborative exploration through representatives from both communities. Through a large workshop, representatives from a number of governmental agencies came together to develop and discuss examples of national interests for civil defence. Continued work following the workshop revealed five key questions, each contributing towards a shared understanding of specific possibilities and limitations. The questions concerned issues of boundaries, contents, importance, location, and value, thereby providing input to a draft sector description to be used when deciding whether to start handling actual national interest claims for civil defence or not. Through the work reported, the paper illustrates recurring challenges with handling divergent and shared meanings, balancing of logical and physical aspects, and bridging of means and ends in collaborative exploration.