The Testimonio II building is located in the Principality of Monaco. Testimonio II includes 348 housing units in two residential towers, a 50-place nursery, and a new site for the 700-student Monaco’s International School. The building has multiple levels of underground parking garages, which are funded on a complex foundation featuring different base levels depending on the location within the site. The global Testimonio II foundation includes barrettes, piles, and diaphragm walls and slabs. Some of these foundation elements were thermally activated to harvest geothermal energy from the ground and power the building together with a vertical geothermal borehole field drilled underneath the building. For this project, GEOEG performed the schematic and detailed designs of the energy foundation as well as the world-first thermal performance tests of energy barrettes that have been built to support the considered building ensemble. Such activities have allowed to quantify and optimize the energy performance and efficiency of the designed geothermal heat exchangers, providing a renewable energy system that massively contributes to the decarbonization of the Testimonio II building ensemble.