Concern for the environment has been steadily growing in recent years, and it is becoming more common to include environmental impact and pollution costs in the design problem along with construction, investment and operating costs. To economically respond to the global environmental problems ahead, progress must be made both on more sustainable technologies and on the design methodology, which needs to adopt a more holistic approach. Heat pumps and, in particular systems integrating heat pumps and cogeneration units, offer a significant potential for greenhouse gas reduction. This paper illustrates the application of a multi-objective and multi-modal evolutionary algorithm to facilitate the design and planning of a district heating network based on a combination of centralized and decentralized heat pumps combined with onsite cogeneration. Comparisons are made with an earlier study based on a single objective environomic optimisation of the same overall model. The advantage of the two objective approach is illustrated with the concurrent minimisation of CO2 emissions and heating cost for the given yearly requirement of part of a city. For the given 62.7 kW thermal rated plant meeting the needs of a set of new buildings with predominantly low temperature heating systems, Pareto curves illustrate the tradeoffs as well as the optimum supply temperature trends. The evolution of the optimum configuration of the central cogeneration and heat pump plant as a function of the CO2 emissions is also shown.