The extension of an existing underground heat pump station was simulated in 3D to identify the impact of the excavations. The assessment included the potential rock mechanics effects on the foundation rock mass of the nearby buildings and a feasibility analysis of the selected excavation method. Simulations were performed with 3DEC using a discrete fracture network model based on the fracture analysis of the site to simulate the behaviour of an actual fracture network. This included the simulation of the different excavation phases which enabled the assessment of the stress changes in the rock mass and the resulting loads and displacements.