Do radiators always have to be placed under the windows in new buildings?

also has radiators and has disguised them so well :)

You certainly don’t save that much. There is still screed, insulation at least on the bottom as well. You don’t save the heating circuit distributor. For that, you need a larger heat pump with heating circuits.
And if you then calculate half a square meter lost for each heating circuit (and theoretically would have to build larger for the same living value), then another €10,000 quickly gets added to the bill.

Better save on the buffer and get exempted from the ERRs. That also quickly saves €3,000-4,000. How to do that is explained in numerous threads here.

also has radiators and has disguised them super well :)

Yes, we have radiators in the middle of the room (behind the slats in the room divider). For us, a space-saving solution and pleasant warmth during the cold months. However, there is still a long radiator under the large window in the living room.

For you, it was a necessary evil because of the "damaged" floorboards, right?

For you it was a necessary evil because of the "broken" plank floor, right?

Correct… because of the plank floor, nothing else was an option for us. I wouldn't call it an "evil" now.. we're doing just fine with it. By the way, in the tiled rooms (kitchen, hallway, and bathroom) we have underfloor heating.

Do you run the HK at the same temperature as the underfloor heating?