## 1 Introduction  
In the European Union (EU), buildings consume approximately 40% of total energy and are responsible for around 36% of CO\({}_{2}\) emissions [1, 2]. Recognizing the need to achieve its ambitious energy and climate goals for 2030 and 2050, the EU identified the building sector as a key priority due to its significant potential for cost-effective energy savings and a corresponding reduction in greenhouse gas emissions [3]. The revised Energy Efficiency Directive (EU) 2018 [4] established a major energy efficiency target of at least 32.5% for the EU by 2030, compared to projected energy use for that year. Furthermore, the EU Commission proposed in the Climate Target Plan 2030 [5] to reduce net greenhouse gas emissions in the EU by at least 55% by 2030, compared to 1990 levels. Energy efficiency is regarded as a crucial element of these efforts, with a particular emphasis on scaling up and accelerating actions in the building sector. To achieve the 55% emissions reduction target,the EU plans to decrease greenhouse gas emissions from buildings by 60%, reduce final energy consumption by 14%, and lower energy consumption for heating and cooling by 18% by 2030. Denmark is no exception to these challenges, as the building sector accounts for a significant 40% share of the overall energy consumption and corresponding greenhouse gas emissions [6]. Consequently, the Danish building regulations, known as 'Danish BR' [7], underwent continuous enhancements in recent decades, primarily focused on newly constructed buildings. These standards encompass strict regulations for building design, operation, thermal comfort, and air quality, as well as the building envelope, components, and specifications. Figure 1 illustrates the progress made in terms of the maximum allowed primary energy use for a typical 150 m\({}^{2}\) newly constructed Danish residential building, highlighting the evolution and development of different building regulations ('BR') [8]. Notably, the permissible primary energy use for a newly built structure decreased by 95%, from approximately 350 kWh/m\({}^{2}\) per year for buildings constructed in 1961 to just 20 kWh/m\({}^{2}\) in 2020. This clearly demonstrates the significant potential not only for constructing energy-efficient new buildings, but also for enhancing the overall performance of existing buildings, particularly those built before the 1980s, through systematic retrofit processes and activities. Building energy retrofitting is a proven and widely implemented technique that demonstrated positive impacts in technical, economic, and societal aspects. However, the annual energy renovation rate in the EU remains low, at only around 1% of the building stock per year [1]. At this pace, achieving carbon neutrality and decarbonizing the building stock would take centuries. Therefore, it is crucial to take action and scale up efforts and innovations. Over the past decade, numerous Danish, European, and international initiatives and strategies emerged, calling for widespread implementation of building energy retrofitting projects, improvement of existing buildings, and reduction in carbon emissions. In response to these ambitious initiatives, this work presents a timely and urgently needed intervention. Its goal is to design, develop, and demonstrate the first-of-its-kind digital twin solution called 'DanRETwin', which will facilitate optimal decision-making, retro-commissioning, and performance optimization of non-residential existing buildings. This work represents the first Danish initiative to leverage digitalization and data capabilities to enhance and accelerate energy retrofitting projects. The digital twin  
Figure 1: The maximum allowed annual primary energy usage in connection to the progressive building regulations for a 150 m\({}^{2}\) Danish residential building [8]. solution integrates real-time data collected from building energy meters and innovative clamp-on IoT sensors with dynamic building energy models to offer a robust and flexible platform.