## 1 Introduction
The cost-optimal method was proposed by the EU Commission for the evaluation of minimum energy requirements on the national level [25] and further developed throughout Annex 56 on the building level [15,26,27]. The method has been applied in varying scope across different building types, e.g., schools [28], dwellings [29, 30, 31], or combination of building types covering a given share of the building stock [15,32,33]. Studies have also established optimization methods for the selection of an optimal solution. It is noticed that such optimizations are more often applied to a complete building renovation or packages [29,31,32], than to a specific component or element of the renovation [34,35]. In addition to the widely applied cost-optimal approach, there are studies that focus on other parameters and/or methods for the evaluation of NZEB renovation [16,32,36]. A literature review presented in [37] focused on renovation methods and criteria, indicating that the majority of the methods compare different packages or various social, economic, and/or environmental indicators, rather than the contribution of the individual elements and systems. This is in agreement with the conclusions reported by [PERSON] et al. [11] for the need of further evaluation of cost-effectiveness and energy savings of each individual renovation measure. An approach for evaluation of the individual contribution of diverse renovation measures was proposed in [37]. The main limitations of the approach were that it disregarded actions related to change of energy source and renewable energy production; moreover, it does not extend to LCC package evaluation. This paper presents an assessment methodology for decision support and the selection of renovation strategies to reach the NZEB standard for a specific building. The suggested procedure considers economic and energy indicators, seeking cost-optimal balance between investments in energy savings and renewable energy production. The economic impact of each improvement to the building is first assessed individually and then as a combination of improvements (packages). The individual assessment is done by a previously proposed cost-effectiveness parameter (CEP) [37], coupling primary the energy savings, implementation cost, and lifetime of individual renovation measures. The CEP is adapted to assess renewable energy-producing systems and used to select cost-effective renovation packages. Packages are evaluated using LCC calculations in accordance with DS/EN 15459-1:2017 [38] and the cost-optimal approach [25], taking into account supply and renovation cost. To establish the validity of the CEP, it is compared to a Net Present Value (NPV) derived by LCC calculation. Demonstration of the method is made using a case study in Denmark consisting of residential social housing complex from 1949, which was previously analyzed in [37]. This contributes to accelerating renovations to NZEB standards by defining the necessary activities, decisions, tasks, and analysis needed to assess an existing residential building and convert it to the NZEB standard by selecting cost-optimal renovation package. The main novelty of the paper lies in the suggested evaluation and selection of which renovation measures to be included in renovation packages.