In this work, a tool to design district heating networks (DHN) is presented and applied to the city of Lausanne as a case study. The evaluation of the buildings’ heat/cooling demand is performed using a Geographic Information System (GIS) database, built from different public databases, national norms, and real consumption measurements. In a first approach, the city is decomposed into smaller districts, then heat and cooling demands of each district are determined, and the investment and operational costs of the DHN calculated using a parameterized empirical formula. The costs of the pipes that connect the districts to the heating source are computed by routing the primary network using the Minimum Spanning Tree (MST) algorithm. This methodology was calibrated for the city of Lausanne, and the influence of the system design and supply/return temperature levels on heat and cooling distribution costs was studied considering the current buildings connected to the DHN.