Is building a house today not payable within a lifetime?

Seriously, why not a small 100sqm bungalow? If I understood correctly, you are alone, maybe a partner will come along later. A nice living room with a dining area and a manageable kitchen, an 18sqm bedroom and a bathroom are enough. You theoretically don't even need a guest WC, that would be a bonus. You end up with totally different costs than if you build two stories and 120sqm+. Then treat yourself to a staircase to the cold attic (you can insulate later) and have plenty of storage space there and don't need a cellar. The rest goes in the shed/garage in the garden.

Why does that depend on the wall thickness?

And that's what you want? So they should be as unobtrusive as possible in the room? Should something else be in front of the window so close to it? Generally, in front of an often used passage leading outside, there should be little standing, no matter what kind of window opening mechanism it is. And whether it's 40 or 50, does that really make such a difference for you? You basically always have just under 40 nowadays.