Thermo-active piles are extensively utilised for providing low carbon heating and cooling to buildings and are a key technology to help fulfilling sustainability targets. However, when deployed as a group, thermal interference between neighbouring piles can significantly impact the overall thermal performance of the system. This study first explores the lower and upper bounds of thermal interference by comparing the thermal performance of a single pile with that of infinitely-large thermo-active pile groups. A simplified method is subsequently proposed to quantify the effects of thermal interference and estimate the thermal performance of thermo-active piles arranged in realistic group geometries. This method involves deriving thermal interaction factor curves that represent the penalty on the thermal performance of a pile due to the presence of another pile in its vicinity. By applying the principle of superposition, the penalty on the thermal performance of any pile within any group can be calculated using these thermal interaction factor curves. The accuracy of the simplified method is validated through comparisons with numerical analysis, demonstrating its ability to estimate accurately the thermal performance of 2 × 2 and a 3 × 3 thermo-active pile groups, regardless of the pile diameter considered.