Construction company wanted in the Stuttgart metropolitan area!

Hauser: because it is easier=faster=cheaper for the installers to lay the pipes on a separate level below the ceiling. In addition, styrofoam is significantly cheaper than concrete, which explains the styrofoam-concrete ceiling. It also saves an additional thermal insulation layer under the screed, or makes it easier. That is why you also do not get a massive monolithic wall from Hauser, because it is cheaper.

And cramming cables into a hollow core ceiling is significantly less fun, much more complicated. And if mistakes happen, it is difficult to access again.

Köhler does it probably like every normal classic solid construction contractor here in the southwest. Ceiling is ceiling and the installation level is located below the screed.

Question to @ : How is the basement planned? Heated, partially? How is the floor slab supposed to look? Classic with screed on top with insulation, or insulation under the floor slab? Swedish floor slab with integrated heating loops? I would pay attention to that as well.

Besides, Styrofoam is significantly cheaper than concrete. That's why you don't get a massive monolithic wall at Hauser.
And cramming cables into a hollow core slab is much less fun, way more complicated.

Do they at Hauser really find their system so galactically irresistible that everything has to be made from it? - you can use ETICS (or also shuttering blocks) for walls and combine them with the same "filigree ceiling" that the porous-brick neighbor uses (?)
I'm always skeptical of "one solution fits everywhere."

Question to @ : How is the basement planned? Heated, partially? What is the floor slab supposed to look like? Classic and then screed on top with insulation, or insulation under the floor slab? Swedish floor slab with integrated heating loops? I would pay attention to that as well.

It is a sloping plot with the entrance/street on the basement level. This means the basement is open towards the front and embedded in the slope at the back. The front rooms (hallway and office) are to be heated by underfloor heating, the rear rooms (basement and technical room) without heating.

Hauser says about the floor slab: "Floor slab 25cm with WU concrete according to DIN 1045 [...] Insulation 80mm WLG 035 of pressure-resistant polystyrene under the floor slab and at the edge of the floor slab. Below that 8cm gravel layer with PE foil on top."
And about the screed: "On the floor slab in building technology a polymer bitumen membrane with aluminum layer, on top insulation 2x40mm rigid foam and 40mm calcium sulfate screed. In the stairwell rigid foam 40mm, underfloor heating stapler plates 20mm and 60mm calcium sulfate screed."

I cannot tell you about Köhler at the moment.

Köhler (and U-Haus similar)
Outside: Ytong aerated concrete ThermStandard PP2-0.35 with 0.09 Lambda
Inside: Silka sand-lime brick
Floor slab: 22cm
Clear shell construction height: 2.65m - finished room height approx. 2.50m

Unfortunately, I do not have any documents from Köhler yet, so this is only from memory and my notes, but I think I made a mistake in the execution above. If I remember correctly, there is still a distinction between load-bearing and non-load-bearing walls.

I think the correct version is:
Outside: Ytong aerated concrete ThermStandard PP2-0.35 with 0.09 Lambda
Inside load-bearing: Silka sand-lime brick
Inside non-load-bearing: 10cm gypsum wall panels

Could that be right?
I would have to find out again.

Do they at Hauser really find their system so galactically irresistible that everything has to be made from it?

Ooooooooh yessssssss, they do