Transforming cities with the aim of achieving cleaner energy targets is a major bet worldwide, dealing with the immense stress of the rapid urbanisation, the depletion of natural resources, the climate change and its impacts during the post-industrialised period. Struggling with this problematic, in this work we develop a horizontal and cross-sectoral process as an integral part of the city planification towards the energy transition. Along with demonstrating the applicability of the zero-energy idea in more universal approach, we validated its feasibility at the case-study Cuesmes (Belgium), a social district with high energy requirements along with the RE-SIZED research project. In response to this, in this work we articulate the feasibility of the concept (zero energy) in urban scale, in a comprehensive toolkit. Comparisons and similarities with previous empirical analysis are discussed as well to complete the work and its findings.