Construction of new low-energy buildings (LEB) areas is attracting attention as a climate mitigation measure. Heat can be supplied to buildings in these areas through individual solutions, through a small, on-site heat network, or through a heat connection to a close-by district-heating (DH) system. The choice between these options affects the energy supply systems and their carbon emissions far beyond the LEB area. We compare the long-term systems impacts of the three heat-supply options through dynamic modelling of the energy systems. The study draws on data collected from a real LEB area in Sweden and addresses scale-dependent impacts on district heating systems. The results show that, generally, the individual and on-site options increase biomass and electricity use, respectively. This, in turn, increases carbon emissions in a broader systems perspective. The systems impacts of the large heat network option depend on the scale and supply-technologies of the DH system close to the LEB area.