This report discusses the need to harmonize the environmental assessment of buildings and provides recommendations for calculating the life-cycle Global Warming Potential (GWP) indicator for buildings. It emphasizes the importance of standardizing data and calculation methods to achieve comparable results. The report also addresses topics such as the inclusion of vegetation on building plots, assessing GWP for existing buildings, and defining sustainable forests for calculating the contribution to biogenic carbon storage in building products. It recommends using a tiered approach and decarbonization scenarios for data used and scenario settings. Overall, the recommendations aim to support a Nordic common implementation of the Energy Performance of Buildings Directive (EPBD) and its climate declaration.