Within the Norwegian Research Centre for Zero Emission Neighbourhoods (ZEN) in smart cities, a definition for achieving zero emission neighbourhoods will be developed and tested against nine pilot areas. The ZEN definition considers a series of assessment criteria and key performance indicators (KPI) under seven categories; GHG emissions, energy, power/load, mobility, economy, spatial qualities and innovation. This paper presents the first draft of the ZEN definition, and discusses some of the opportunities and challenges in implementing assessment criteria and KPIs (ZEN metrics) into a ZEN KPI tool. This paper briefly presents the ZEN pilot areas and maps out existing tools used by ZEN stakeholders for the documentation of ZEN metrics. The paper goes further by presenting a ZEN KPI tool conceptual framework for the implementation of the ZEN definition in ZEN pilot areas and outlines a specification for the future development of a ZEN KPI tool. Finally, the paper presents further work for the development of a ZEN KPI tool.