Floor plan tube house L-shape triangular plot including oak tree

... but you probably also consider the ideas of an underground garage and a granny flat in the basement either impractical or feasible but more than strange - right?

I have now shifted the tree circle a bit more towards the east, but only by 7 m plus 1.5 m. The space will actually become even tighter there.

It would be good to know how far this oak tree is from the east side and whether it really stands exactly on the property boundary.

So you know the notarized contract, you know what the OP paid and what the seller told or didn't tell?

Believing does not mean knowing.

Oh, so I still have to make an offer?

That's weak. So, for honor's sake. Okay!

Good grief, Katja! Google Maps! And where did you get the property boundaries from?

Just take a look! You can see them.

Your support is honorable, but now just think about what the OP's wishes were and what remains of them here. A life in a tube. Do you really want to advise him that? You have to see that at least most people spend their entire life paying off their property. And the OP is supposed to do that for this, whatever it may end up being? I'd rather advise him to build a public toilet there and charge one euro per person.

I found a report last night about the construction area next door. Of the 116 trees that fall under the building regulations, only 3 are supposed to remain. Compensation measure: new trees and money. Surely a solution can be found there as well.

The only possibility for a sensible building, I think. But will it work?

The floor plan of Kbt was not that narrow after all. I had imagined it worse. Many live on 3 levels. I don't find that bad. From all rooms, there is a view of the tree. Below is the terrace with natural sun protection and a nice climate thanks to the tree.

It would be good to know how far this oak tree is from the east side and whether it really stands exactly on the property boundary.

Exactly that is the point. And for that, the OP also deserves criticism for not providing clarity here and not revealing his ideas regarding the building structure and location on the property. But the way this criticism is presented here is really below par:

undevelopable

nonsense

to vomit

lame

to rip off

stubborn idiot

... and all that with unknown or wrong measurements and unclear minimum distances to public traffic areas.

Just take a look! You can see it.

That was a good one!

... but you surely also find the ideas of underground garage and granny flat in the basement either unfeasible or feasible but more than strange - right?

Hard to say. I’m not much of a design talent. "Underground garage" – the property rises by one meter to the north. So instead of 2.50 you only have to go down 1.50. You would have a half-story towards the street. Whether lighting requirements can be met there is questionable but not impossible.

...the way this criticism is presented here is really bottom of the barrel:

The quotes are for the most part completely taken out of context. I do not consider protecting someone from making a big mistake by telling the truth to be "bottom of the barrel."
Anyway. If there is a plan, I will definitely support the original poster, if they wish. So far, however, I continue to advise against forcing it through at all costs.