Purchase or construction in Berlin / surrounding areas, realistic?

- Underfloor heating or radiators (radiators are 5k cheaper, I still need to read up on whether one or the other differs in energy terms over the long term?)

No, stylish (vertical) radiators are insanely expensive. And heating costs are even lower with underfloor heating thanks to the low flow temperature.

I imagined something like what I currently have in the rental apartment for radiators and saw the price difference to underfloor heating in the fact that it might be less effort. And yes, they are not pretty, but I am currently living with them anyway.

So reliant on rental apartments that you needed air conditioning?

Not on attic apartments, if you ever lived in one. But on "normal" ones, yes, heavily.

The air conditioning didn’t cost much, but it’s not representative since it was self-installed. But from about 2K per room (including installation and commissioning) you’re looking at that.

The property has no natural shading because, as so often, it’s a new building and no tree population. With forest around it would look different, but then you’d have less light inside the building.

What specific advantages does underfloor heating offer?

Lower operating costs, more pleasant warmth, no cold spots/floors/corners in the house, more usable space, etc. The standard radiators you’re considering I wouldn’t want in my own new build. That’s something for rental properties.

A fireplace means more work and costs a lot of money. And then the thing is only on 2-3 times because it often gets too warm in a modern building, and I think spending money for that is wasteful times ten.

Attic not allowed, so far max. 4th floor. We always had the windows open in summer, so more or less outside temperature, but not warmer. 2k per room is already quite decent. Depending on how many rooms need to be air-conditioned. Then it's almost worth building a basement. The gentleman can cool down downstairs and I enjoy my 35 degrees on the terrace. Lack of trees is a big disadvantage, that's true. In my experience, willows grow quite quickly and easily, but it still takes a few years. How is pleasant warmth defined? Sorry that I have to ask such a silly question, the last time I was somewhere with underfloor heating was quite a while ago. I am currently sitting with my back to the radiator and working sitting on the floor because I am cold...

For my husband and me, it was very instructive to live between the old house and the new house in a rental apartment and to realize how good we are here in the rental apartment

That is wonderful and also corresponds to my experience - my wife and I suffice for each other. What we have in luxury is pure cream - tasty but unnecessary. The discrepancy between what we need and what we want is often very large, and the ability to distinguish between the two is often very foggy. How could people live without a dressing room in the past? They couldn’t, they all died. :D

The marked kitchen area seemed too small for our purposes, but the open floor plan still allows, for example, adding a central island. If we want that, we have to think about connections.

As a rule, the designers have a reason for drawing the kitchen layout the way they did, and it’s the same here. If you enlarge the kitchen with an island, you will hardly have space for the dining table. I would only extend the leg of the L at the top of the plan. But I hardly see an island as feasible.