Life cycle oriented methods are increasingly used for environmental assessments (EAs) of the built environment. However, many assumptions are made in such assessments, potentially influencing the results and making the assessment more ambiguous. To increase the reliability of EAs, consequences of the assumptions made have to be better understood. Since energy use in the operation and maintenance phase is an important factor decisive for the overall environmental performance of a building, the purpose of this study is to investigate how the selection of heat and electricity mix affects the assessed environmental performance of buildings. It also aims to suggest how to choose heat and electricity data in EAs of the built environment in general. Applying four different modes of electricity production and two different modes of heat production in a case study of three different buildings with different technical solutions for heat and electricity supply, the study show that choices of heat and electricity mix have significant influence on the final results of the EA. Regarding the choice of heat and electricity mix in an EA of buildings and the built environment, it is argued that both average and marginal data on electricity production should be used in general. As for data on district heat production, it is recommended to use data on the average production in the specific, local district heating system in general. Finally, it is argued that consequences of the assumptions made should be explicitly communicated in the EA report, so as to let the decision-makers rather than the analysis make the evaluation.