Against the backdrop of continuously rising energy prices and discussions about climate change, the existing building stock is becoming increasingly important as a market segment for the construction industry. This study examines energy weaknesses based on a residential building and identifies potential for reducing energy demand through a current analysis that takes into account legal frameworks. Space heating and hot water preparation are the focus of the investigation. The energy performance assessment for the building is conducted realistically based on available regional weather data. Obvious savings potentials exist in the existing structure, both in the building envelope and in heating technology. The energy savings mandated by legislation allow the planner the freedom to combine various construction measures and technical innovations in such a way that the building is optimized.