Experts in the Netherlands have lately debated the novel policy idea to freely apply municipal solid waste incineration bottom ash (MSWIBA). In this paper, we map this ambivalent and unforeseeable, subjective, expert debate. This will help policymaking because more knowledge on subjective expert viewpoints and perceptions allows for clustering conflict and consent as well as gaps in common understanding in this complex debate. We applied Q-methodology which resulted in four distinct perspectives in the expert debate that reveal insights into the social licence to operate and into the legitimacy of the novel bottom ash regime. The freely applicable quality of BA itself is accepted in all perspectives. Conflicting views were found about new risks, trust and socio-political acceptance of the novel BA applications. An important practical contribution of this study is that a higher acceptance of the freely applicable MSWI BA quality in the Netherlands within the expert community can be achieved if the new risks of the free application regime are tackled. We are the first to use Q-methodology in this field, and our academic contribution is that we show that this method can be a helpful tool to unravel complex expert debates also related to MSWI BA applications.@en