So we are currently also building a 55 standard house, the shell is already up and our heat calculation came up with the following:
14 cm XPS insulation under the slab
25 cm base slab
6 cm polystyrene insulation
6 cm screed
If, like us, you want to use the rooms in the basement not only for storage but also, for example, as an office or guest room, I would recommend insulating the base slab. It should improve the living comfort alone. Whether that is economically sensible, however, is another matter. You can certainly also achieve the 55 standard house if you improve other components, but then you would have to, for example, insulate the roof better or install a ventilation system. There are many ways to Rome, uh KfW.