Scenario definition for the built environment at regional and national scale. Following Switzerland's commitment on the path of the energy transition, the new Energy Act [BFE (2017)], which defines the first package of the Energy Strategy 2050 (ES 2050), sets energy efficiency as one of the main pillars of this transition. Within the new Act, the Confederation and the Cantons are expected to adopt plans in the built environment in order to exploit through retrofits as much as possible the potential of energy efficiency, renewable energies, and waste heat. According to OFEN [2012], the building stock consisting of 1.64 million buildings (among which 1.36 million are residential buildings) in Switzerland accounts for 46% of the total national final energy demand2. By 2050, the new energy policy targets a decrease of energy use in buildings by around 35 TWh compared to the trend (baseline scenario) as defined by the energy perspectives [Kirchner, A, & al., 2012]. By 2035, the first package of measures, which came into force in 2018, should achieve a reduction by 23.4 TWh (67% of the 2050 perspective). Energy demand forecasting models are highly dependent on the different trends related to socio-economic factors, such as population, GDP growth, fossils fuels prices and the level of activity in the industry & ser-vices sectors. In view of the evolution over time, forecasts should reflect as much as possible the current economic and social circumstances. This is also valid for the built environment, where the characteristics of the building stock (age of buildings, energy sources used for space heating and hot water) as well as the current and future regulation on energy retrofits and for new constructions have major impacts on the energy demand. The aim of this report is to provide fundamental assumptions influencing the evolution of energy demand in the built environment until 2050. Such common assumptions could be then used by the different FEEB&D participants in the framework or their research, they would help to harmonize the underlying hypotheses when evaluating the contribution to goal achievement under the ES 2050. In this report, information and data from various sources (e.g. OECD, BFS, BFE, BAFU, IEA, Prognos) were collected in order to define scenarios. These data originate from various sources. Not only does this report provide an inventory of existing forecasts regarding the socio-economic variables, it also focuses on relevant assumptions for the built environment based on federal office data or data published by research groups. For example, our report provides information on energy sources and the energy demand of residential build-ings as well as on the so-called energy performance gap. The report also presents a preliminary approach to determine more accurately than so far the energy reference area of residential buildings using various explanatory variables. Simultaneously to this report, the Joint activity and Scenario Modelling (JA&SM) developed a set of scenar-ios. For the purpose of comparison, the relevant information is also included in the current report. A harmo-nized set of scenario definitions favours the collaboration within the research community. The JASM scenarios use the GDP and population assumptions from the ARE [Bundesamt für Raumentwick-lung] scenarios3. To get a consistent dataset with the other socio-economic assumptions needed for the definition of the scenarios (such as sectoral gross value added, production indices or heating floor areas),the JASM team developed a reduced-form econometric model4. The model is calibrated to the correspond-ing assumptions in the Swiss Energy Strategy scenarios from Prognos5, and it uses only GDP and popula-tion as inputs.