This study proposes a novel framework, modular participatory backcasting (mPB), for long-term planning in the heating sector. The mPB framework is based on participatory backcasting (PB) and integrates principles of modularity, participatory modelling, and transdisciplinarity. We discerned for mPB 13 modules that can be arranged according to the purpose and specifics of each planning process. The design of the mPB framework and its implementation are presented for the cases of participatory strategic planning processes to achieve sustainable heat provision by 2050 in a Ukrainian city (Bila Tserkva) and a Serbian city (Nis). The results show that mPB allows adaptability to local contexts and limitations through exclusion, augmentation, substitution, splitting and inverting properties of modularity; decreases the learning time for applying the framework in a novel context; increases the reproducibility and transparency of long-term energy planning processes; enables efficient integration of quantitative methods into the participatory process; and advances collaboration between academia and society. The proposed framework is beneficial for advancement of local planning and policy-making practices by creating strategies with a wider support of stakeholders. It could also be useful for further research through cross-case analysis.