Within the framework of sustainability, the United Nations' Sustainable Development Goal 13 (SDG 13) is to limit and adapt to climate change. The Deep mixing methodology by itself can reduce the climate impact for producing geotechnical works in ground conditions and boundaries where a classic method based on excavation and backfilling with concrete would otherwise have a larger carbon footprint. However, this paper presents another beneficial use of a soil mixing application where such retaining walls become a permanent and integral part of a sustainable building concept. A case study in Bavaria, Germany, demonstrates that the mixed-in-place method is a preferred sustainable technique for harnessing geothermal energy due to favorable heat transfer properties. In this way, the supporting structures, which are often used temporarily, become an integral part of the building and contribute significantly to its energy demands. If such a “geothermal option” is implemented, the CO2 emissions saved can be recorded as the contractor’s handprint on the client’s sustainability account.