## 4 Discussion  
The suggested methodology gives the opportunity to consider a wide variety of necessary activities and select a renovation strategy based on a structured approach. The different stages link regulatory and computational considerations for the comparison and selection of renovation actions, considering energy demand and costs. The methodology is demonstrated using case study buildings that require extensive renovation and energy savings (80% reduction) to reach the NZEB standard. The exiting state of the selected case study provides the possibility for the application of a wide range of energy-efficiency measures and requires a substantial energy reduction. Thus, the selection of which building parts to be renovated and to what extend is crucial for reaching a cost-optimal NZEB solution. The case study results showed a number of different ways for reaching the NZEB target. Furthermore, the applied method helped to identify solutions that have the potential to reach the NZEB target with significantly reduced cost. While the targets for the specific project at hand can vary, the method is refined for renovations targeting the NZEB standard. Currently, an NZEB is defined in the EPDB [7] as \"[a] building that has a very high energy performance, as determined in accordance with Annex I. The nearly zero or very low amount of energy required should be covered to a very significant extent by energy from renewable sources, including energy from renewable sources produced on-site or nearby.\" In most Member States, the limit value of how low the energy demand of the building needs to be is defined by the maximum primary energy. The limit value varies greatly from country to country in both magnitude and the way the indicator is determined. In some Member States, the indicator is a non-dimensional parameter relative to a reference building [8]. In a few countries, carbon emissions is the main indicator, while in others, emissions are used as a secondary indicator to primary energy [8]. Since the release of the EPBD, recast various definitions and calculation methods for NZEB have been applied [46]; moreover, the definitions are still evolving and changing. Therefore, the fact that the methodology is not tailored to a specific limit value but rather focused on activities to acquire and evaluate the parameters in question is considered as a strength. For the demonstration of the method in this paper, single actions are evaluated based on cost and energy savings. The suggested approach is flexible toward the specific energy modeling method. While the presented work adopts a simple, standardized approach, it is possible to be done using other calculation methods tailored to the project needs or local requirements. The accuracy of derived energy savings is dependent on the selected method and accompanying assumptions. Similar statements can be made for the estimation and calculation of costs. Nowadays, this is a much less regulated topic compared to energy performance, but it can be noticed that this is changing with the implementation of new emerging standards [38] and expected EPBD revision before July 2021 [47]. The suggestion for addressing costs, first on a single element level and then by LCC, is in line with a number of the key principles communicated in the Renovation Wave [6]. Addressing single actions in a simplified way may also help bridge the gap of practitioners understanding life cycle thinking. Regardless of the applied approach, any cost results will be dependent on the depth of the analysis and the sources for prices of the different actions (database, specific tender offer, selected websites/companies, product catalogues, etc.). As discussed in Section 2.2, some cost reductions may also occur as a result of the contents of a renovation package. Especially for packages with large energy savings, the total cost may be reduced due to synergies of the performed actions and reduction of the required system capacity.