Districts have a significant role in achieving the principles of sustainability. Within the past decades, a great variety of assessment tools and methodologies has been developed in an effort to ‘translate’ the sustainability criteria into applied cases. There is an increasing interest in this contribution scaled up the assessment to larger territorial analysis and urban agglomerations. Notwithstanding, developing an assessment tool with sustainable standards requires strategic approaches to incorporate the theoretical framework to their implementation of city districts by measuring their performance in a consistent manner in respect of multiple criteria. Among these issues, energy efficiency and the zero energy objectives are significant for European policies. This study aims to provide an overview of the existing assessment tools and methods comparing their criteria and key parameters. As a second step, it introduces a simplified methodological assessment theoretical tool (U-ZED) by focusing on the commitment towards the zero energy targets in a future district. In a more general perspective, the study deals with the challenge of the development of a tool from building to district with the main concern to define the context of sustainable and long-term districts dealing with the challenges of 2050 horizon.