Sustainability in cohesion with real estate investments is still at its initial stage. However, the topic has become more and more attractive over the last years within the field of research. A few prior studies have shown that companies, who act according to an ESG-Agenda, are characterised by lower levels of specific risk. On the basis of these results, this study provides a basic investigation of the contribution of sustainable real estate to an investorís portfolio. Isolating companies who implemented an ESG-Agenda within their long term strategy, we analyse their financial performance characteristics in order to examine the dynamics of sustainable real estate within a multi-asset portfolio. Out of this reason we develop an Index consisting of companies who operate in the real estate sector and act in accordance with an ESG-Agenda. We apply this Index to a mean-variance framework within a multi-asset portfolio and determine the optimal proportion of sustainable real estate considering varying risk and return profiles. The overall results should give an indication of the opportunity of diversification within a multi-asset allocation.