The expertise of impact assessment practitioners and the legitimacy of their reports are increasingly being questioned. We analyze the subjectivity of impact assessment by exploring how the framing undertaken within Heritage Impact Assessment (HIA) influences decision making about spatial development projects. Framing is the process by which actors order and make sense of social reality. We argue that framing influences the content, conclusions, effectiveness and legitimacy of impact assessment reports. We examined a major urban redevelopment, the Liverpool Waters project, for which three HIAs were commissioned to assess its impact on the Liverpool Maritime Mercantile City World Heritage site. These HIAs had varying outcomes with differences in baseline information, variables considered, methods used, and assessment of impacts. In our in-depth interviews with practitioners and decision makers involved with these HIAs, discussion of legitimacy centered around assumed differences between local and non-local knowledge. We argue that awareness of the role of framing is needed in the impact assessment field, and that transparency and participation by local stakeholders are crucial to prevent framing from having an undue influence.