Abstract Many cities leave a considerable thermal footprint in the subsurface. This is caused mainly by accelerated heat fluxes from warmed basements, pavements and buried infrastructures. Even though rough estimations of the theoretical heat content in urban ground exist, there is no insight available on the technical potential of such subsurface urban heat islands. By considering borehole heat exchangers (BHEs) for geothermal exploitation, new opportunities arise for planning sustainable systems within cities through utilization of accelerated ground heat input from urban structures. This is feasible at moderate heat extraction rates even without any active (seasonal) recharging of the BHEs. For typical conditions in central Europe and a given system’s life time, each additional degree of urban ground heating could save around 4 m of the borehole length for the same heating power supply. We inspect implications for a single BHE as well as complete coverage of cities, which is approximated by an infinite field of BHEs. The results show that shallower systems favour renewable operation, and urban technical potential of geothermal use increases by up to 40% when compared to rural conditions.