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

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When I see the beautiful solution you have, I would also prefer to have radiators. Now I have to ask the question that has been on my mind for a while: Did you design the interior yourselves or did an interior designer do it? And if you designed everything yourselves, where and how did you acquire the experience, or were you simply born with excellent taste?

Rethinking the mainstream routine is "more dangerous" than what can be "saved" in procurement.

Among other reasons, I gave a like for his post. I find it important to question and examine even seemingly "universally valid" things: what do I want/need and does a seemingly suboptimal solution have advantages that outweigh the disadvantages for me. The weighting of pros and cons is subjective. That’s why this forum is so great. :) Here you encounter an enormous amount of knowledge, experience, information, and different opinions.
I admit I am not neutral, because although we are building a completely ordinary house, we have partly deviated from the mainstream; that is probably why the concerns of the "deviators" are so important to me. ;-)

We are afraid that the underfloor heating has a leak and something will have to be replaced in 10 years because of it.

I understand this concern, as I have also heard it several times myself. It would be interesting to find out how often this happens statistically and whether this concern is still justified nowadays or comes from "earlier times." Also, what solution options exist today for a leak in the underfloor heating.

The chances of water damage in water-based heating systems are probably the same. We had the pleasure that the pipes in the wall had a leak. Back then in the shared apartment during university. It was only noticed because the landlord had to refill water - eventually he wondered where the water was seeping to. We and the neighbors below had to move out for the renovation. Anyone who is afraid of water damage should heat with electricity. :)

oookkkay.. So no screed and no underfloor heating to save costs. Basically: underfloor heating compared to "normal" radiators saves exactly nothing, zero point zero. My brother-in-law is a technical building equipment planner and in larger projects that are sometimes built "socially," radiators are installed instead of underfloor heating exclusively!! to technically present these apartments differently (worse). In the end, they are not any cheaper by even one euro. Later on, in terms of consumption, of course not either, on the contrary.

But: If you want to save money and don't want screed: Just do it yourself. On the raw floor, you make (from here please all by yourself since it is simple and only then the savings effect occurs) a substructure, on top of that e.g. parquet (second choice from the outlet). Under the parquet, insulate everything nicely with mineral wool. Done. That's how I did it (not necessarily for financial reasons, but it was still cheaper). However, I don't have conventional radiators but wall heating. If I were to build again, these would also be done by myself.

There are "click systems" that are so simple. If something becomes leaky: tear open the wall, patch it, close the wall again. Not nice but doable. Or infrared heating, there is no water flow then but requires a rather generously dimensioned photovoltaic system which is not fundamentally against cost savings but requires liquid funds first ;)

Which heating system is planned? Heat pump, gas, …

Are there still window recesses in new buildings? Mostly, the walls are built with the same thickness everywhere for thermal insulation purposes. However, I would also say that the approach is (unfortunately) going in the wrong direction! The energy carrier for >40°C flow temperature is becoming increasingly unappealing, and costs will continue to rise in the future. Exhaust air heat pumps not an option?