## 2 Methodology  
In terms of climate zones, building styles, and usage, Europe's building stock is quite diverse. In fact, even within the same category, building usage might vary significantly between Member States. Climate conditions have an impact on construction methods and the energy requirements of a building. More specifically, the information gathered for the creation of RBs can be conveniently divided into four categories: 1. form: building type (e.g., office, school), size, and overall geometry of the structure; 2. envelope: the building's construction techniques and materials; 3. system: HVAC systems and renewable energy production; 4. operation: operational parameters impacting building utilization. Furthermore, acquired data are categorized according to age, location, and type. [PERSON] et al. (2013) [26] state that there are three methods for classifying RBs:  
1. \"Example (Reference) Building\". When no statistical data are available, this approach is applied, and it consequently depends on expert opinions and research. The end result is a structure that is the most likely among a set of buildings in a given location and age. 2. \"Real (Reference) Building\". The RB is the most common building in a certain class. It is an actual, existent building with statistically average qualities. 3. \"Theoretical (Reference) Building\". This technique uses statistical data to define a reduced set of RBs as a statistical combination of the attributes observed within a building category in the stock. As a result, the structure is constructed using the most commonly used products and components. The decision between these alternatives should be based on professional advice and the availability of statistical data. Different approaches can be used for various construction groups. Building typologies are an effective tool for gaining a thorough understanding of the energy performance of various building forms and categories. Residential building typologies for seven European Mediterranean (MED) nations were evaluated in the scope of the HAPPEN project (H2020-EE-2016-2017) using a similar methodological approach. Each national typology comprises a categorization scheme that classifies buildings based on their size, age, and other energy-related criteria. Determining the cost-optimal renovation scenario for each individual building is an almost impossible task to perform, and therefore, in order to overcome this difficulty, an RB can be used to reflect the typical and average building stock in a given nation. As a result, the developed RB should reflect the real national building stock as closely as possible so that the approach may produce accurate calculation results. The essential data for each MED country addressed by the project were obtained from previous research and other EU initiatives, as well as from the expertise and experience of project partners. In circumstances where the necessary data cannot be retrieved, the use of default data is advised. This information is necessary for the corresponding clusters to be formed, for example, by building size (single/multi-family houses) and/or certain age bands, in order to be able to identify different subsets of building stock. Based on past EU initiatives, as well as regional research and expert knowledge, each partner gathered and evaluated data addressing the general features of heating, DHW, cooling, construction, and geometrical details relevant to all types of residential structures. The selection and formulation of relevant indicators is critical for properly monitoring energy saving efforts in building stocks. The indicator system must be appropriately chosen to map the status of the building stock at a certain time period as well as understand the dynamics of evolution. Basic and structural data are necessary for the establishment of building stock models for energy balance calculations.