The transition toward greener district heating (DH) systems is supported by the low-temperature operation of building heating systems. In addition to reducing the DH supply temperature it is necessary to parallelly decrease the DH return temperature. A common bottleneck in lowering DH return temperatures are multi-apartment buildings operating with domestic hot water (DHW) circulation loops. The most common substation design in existing systems heat the DHW circulation using the DHW heat exchanger (HEX). However, as the DHW circulation return temperature is high and the DHW circulation energy demand is relatively high as well, it often results in high DH return temperatures from the building. To address this challenge, this study investigated an innovative design for future-proof DHW substations for large multi-apartment buildings. In the new design, the DHW and DHW circulation loop are decoupled, each utilizing a dedicated HEX for its specific purpose. This new design enables aftercooling the high DH return temperature from the DHW circulation by channeling all, or part, of the return water through the space heating HEX. For the building case examples presented in this study, the DH return temperature reduction potentials are in the range of 5.4 °C–8.3 °C for the 4G temperature profiles.