Determining ‘valid knowledge’ and ‘the facts that matter’ are key concerns in planning practice and literature, but much existing theory places agency on the planner to determine validity while overlooking the structural mechanisms that surround complex decision-making in planning practice. This research applies the philosophy of critical realism to identify key elements of structure including a critical realist conception of structural hegemony, alongside political rationalities, public policy doctrines such as the public interest, and professional ethics. These concepts are proposed as key causal mechanisms impacting upon decision-making and the determination of valid knowledge. Referencing an empirical case study of the siting of a waste-to-energy incinerator in the UK context, the research identifies a hegemonic lag whereby hegemonic rationalities and doctrines actualise decisions that endure through subsequent planning processes despite political and public contestation, and changes in policy, local context, and planning personnel. This hegemonic lag is proposed as an explanation of why complex planning decisions often become controversial.