Heritage planning, as an integrated approach to dealing with traces of the past in the ongoing organisation of the landscape, must be a trans-disciplinary endeavour. Bridging differences between scientific disciplines, as well as sciences and the law, administration, politics and economy, is a continuous challenge. We argue that Niklas Luhmann’s social systems theory, with its sophisticated understanding of society as an evolving population of social systems, is very useful in understanding the value and difficulty of trespassing boundaries in heritage planning, and in understanding the value of conflict and cultivated difference in the planning process. We reflect on the mechanisms of self-reference and self-reproduction that are at play within the scientific disciplines addressing ‘heritage’, and analyse similar mechanisms within planning administrations. These mechanisms are not in essence negative; they are necessary for the production of the kind of knowledge that is specific for the system or organisation. However, in planning, some form of coordination of interests and types of knowledge is seen as desirable. We argue for an approach to heritage planning that avoids self-reference in the planning system as a whole, while accepting and cherishing the self-reference of the actors.