With buildings around the world accounting for nearly one-third of global energy demand and the availability of fossil fuels constantly on the decline, there is a need to ensure that this energy demand is efficiently and effectively managed using renewable energy now more than ever. Most research and case studies have focused on energy efficiency of ‘new’ buildings. In this study, both technical and financial viability of Net Zero Energy Buildings (NZEB) for ‘existing’ buildings will be highlighted. A rigorous review of open literatures concerning seven principal areas that in themselves define the concept of NZEB building is carried out. In practice, a suitable option of the NZEB solutions is needed for the evaluation and improvement for a specific geographical area. The evaluation and improvement has been carried out using a novel hierarchy-flow chart coupled with a Building Information Model (BIM). This BIM or digital twin is then used to thoroughly visualize each option, promote collaboration among stakeholders, and accurately estimate associated costs and associated technical issues encountered with producing an NZEB in a pre-determined location. This paper also provides a future model for NZEB applications in existing buildings, which applies renewable technologies to the building by aiming to identify ultimate benefit of the building especially in terms of effectiveness and efficiency in energy consumption. It is revealed that the digital twin is proven to be feasible for all renewable technologies applied on the NZEB buildings. Based on the case study in the UK, it can be affirmed that the suitable NZEB solution for an existing building can achieve the 23 year return period.