We explore whether local planning culture influences the effectiveness of heritage impact assessment (HIA) and we discuss the legitimacy of ICOMOS, the international advisory body to UNESCO on cultural heritage. We examined the HIA processes for two proposed infrastructure projects that potentially could affect the Defence Line of Amsterdam World Heritage site in the Netherlands. We interviewed key stakeholders involved in decision-making about these pro-jects, and found that the Dutch planning culture positively influenced the effectiveness of the HIA processes. The interviewees predominantly discussed the substantive and transactive effectiveness of the HIA processes in that they praised the practitioner for facilitating a clear, inclusive and transparent process and having a solution-oriented mindset, which is common practice in the Dutch planning culture. However, in contrast to the Dutch planning culture, the role of ICOMOS was perceived as opaque and a ‘black box’, although this did not decrease its legitimacy among the key stakeholders.