The design outdoor temperature is determined using local climate data and describes the coldest two-day average that has occurred at least once every two years over a period of 18 years. The design outdoor temperature is specified for Germany to the exact postcode and is – 11.2 ยฐC for Berlin-Mitte (postcode 10178), for example, and can be viewed in DIN EN 12831. If you do not have DIN EN 12831 to hand, you can determine the design outdoor temperature for your location from the climate map of the German Heat Pump Association (only for germany).
Further links and sources:
DIN/TS 12831-1 – Method for calculating the space heating load – Part 1: National addenda to DIN EN 12831-1