The use of local knowledge adds value to the decision-making process, for which Public Participatory GIS (PPGIS) are widely deployed. However, there are often issues in the way that PPGIS are designed, particularly with respect to the type of spatial representation used. We propose ‘informed interfaces’ as a novel approach to PPGIS interface design, to ensure the system can effectively reflect the interests, priorities and values of participants in case-specific spatial decision-making. This paper introduces the concept before demonstrating the benefits of the approach using two examples of informed interfaces through an illustrative UK case study. Evidence was gathered from three face-to-face workshops and five multi-participant online usability tests, revealing that participants felt more confident in the datasets they produced using the informed interfaces. The results also confirm that informed interfaces hold the potential to provide richer, more veracious datasets for improved decision-making, revealing new insights into local perspectives.