Screed execution in the attic - ceiling between the attic and living space

is it then sensible to insulate under the screed?

You can do that. Then the pipes are not directly in the screed. But as I said, your entire house is insulated. Like a refrigerator. If you put an additional compartment in there, it still doesn't change the temperature in the entire unit.

My question is whether I lose too much heat here in winter because I heat the storage.

My question is whether I don’t lose too much heat here in winter because I heat the storage.

If 30€ heating costs per year are too much for you, then you have to completely remove the attic from the equation. So properly insulate the ceiling - I would then do 30cm of insulation. But then it will probably eventually get cold in the attic and maybe also damp...

I wouldn’t do it. I would put a few centimeters of insulation under the screed so that the pipes and conduits are covered. Do you even have enough space to sensibly construct the floor structure?

You are probably thinking of grandma’s little house where the attic is freezing cold and the heat from the upper floor is lost. But that is not the case here. You lose some heat from the upper floor into the attic but only a little from the attic to the outside. After all, your attic is inside the thermal envelope if I understand the original post correctly