In this work an optimisation based approach for the equipment arrangement planning of a microgrid and the path of the hot-water piping network for supplying the excess heat of different microgrid technologies to each building, under the cost minimisation objective, is developed. Taking into account electricity and heat load profiles of the different buildings of the microgrid, utility tariff structure, characteristics of the candidate DER technologies (technical and financial) as well as geographical aspects, the mathematical model selects the optimal system configuration as well as the design of the heating pipeline network. As an illustrative example, the optimal DER system of a neighbourhood of five buildings, in Athens (Greece) is examined under different scenarios, thus demonstrating the applicability of the proposed optimisation based approach for "what-if" analysis.