How many degrees of heat loss per night is "normal"?

The heating will be an air heat pump with 5.6KW

One could - if one feels like it. I know several wooden house builders who have been heating for years without a "conventional" heating system. As I said. You have to feel like constantly adding wood and it can't be regulated very well. But it does work and it is allowed :P

But then connected to the underfloor heating, right? Otherwise, the uniform heat distribution will always be a problem