Underfloor heating -> Structure of the subfloor?

Good evening! We urgently need advice. We have already read up on the internet but have come across different statements: Mid-terrace house, built in the 60s, floor removed down to the concrete. We want to install underfloor heating... have heard about these underfloor heating systems as a dry construction system. But basically, we have the following question: - how is the structure of the subfloor composed? Subfloor fill - insulation - underfloor heating - screed - floor covering? - why does the underfloor heating actually have to be under the screed? Would it also work above the screed? - is there anything else important to consider? Thanks in advance for the answers

- why does the underfloor heating actually have to be under the screed? Would it also work above the screed?

That has many advantages: the screed protects the heating, distributes the heat, and stores it

Sounds logical and comprehensible

We want to install underfloor heating... have heard of these underfloor heating systems as a dry construction system.

Why dry construction? Where is the underfloor heating supposed to go? Insulation can go under the underfloor heating, but also under the basement ceiling as an alternative (then you don't lose room height). Tell me more about what and why you are planning.

Why drywall? Where is the underfloor heating supposed to go? Insulation can go under the underfloor heating, but also under the basement ceiling as an alternative (then you don’t lose any room height). Tell me more about what and why you are planning.

But then we would have to lower the basement ceiling, right? We didn’t want to do that additionally. Do you mean in which rooms the underfloor heating should be installed? In all living rooms. The drywall system should be considerably cheaper. For 100m2 about €6500, that’s why we want to decide on it. But as you can tell, we don’t really know much about it.

Drywall system is supposed to be considerably cheaper.

I think that's a rumor.

Then we'd have to lower the basement ceiling, right?

Yep.
Basement heated? How is it supposed to be insulated under the underfloor heating? How much space do you have? ==> that it should be properly insulated is beyond question. Otherwise, you are basically heating the basement a lot.
"Lowering" means, for example, that you buy 60/80/100mm PUR aluminum foil-coated insulation. Along with PU adhesive foam, just stick the boards to the ceiling. That is very cheap, costs a few hundred euros, and brings a lot.

Have you already had the heating system designed? Room-by-room heating load calculation as well as specification of the desired room temperature. Make sure you get the supply temperature as low as possible.

Do you mean in which rooms the underfloor heating should be installed? In all living rooms.

Why not in all rooms if you're already at it?