Insulation of a detached semi-detached house - experiences?

With an additional 1m you will cause yourself difficulties. Then the construction of the second semi-detached house will no longer be possible due to the boundary location -> so in the worst case the distance regulation will apply. This means you will need a building encumbrance with the neighbor again (boundary distance < 3m).

But now I insulate with 16cm

As I said; however, this may not be necessary at all. Actually, the KfW standard must be achieved even without a second semi-detached house standing there.

Who brought this problem into play? You, or the builder?

How about buying 3m off. Then you really have a detached house and could also add windows to the wall if needed.

You are not doing anything at all.
6cm insulation, plaster, done.
If the investor says it will be KfW55, he will already have some energy consultant who calculates it that way for him.
Whether there are 6 or 16cm on the wall, you won't notice.
Why does it even have to be KfW55? Is funding dependent on that? If not, it doesn't matter anyway.
Be happy that you paid for a semi-detached house plot and probably got a single-family house plot (with a small restriction).

I don't think they are necessarily looking for a "fool." The builder or investor simply made the mistake of starting construction on the first half without a binding agreement from the neighbor for the second half of the property. The building permits for both halves are to be viewed separately.

Okay, I correct myself: he was the fool, and is now looking for a second fool.

For interested buyers like me, there is now the chance to get a nice detached house.

That can come of it if you follow this advice ...

How about buying 3m off. Then you'll really have a detached house and if needed, you could also add windows into the wall.

... with a small variation, take a bit more, for example because of insulation and a somewhat more comfortable parking space, let's say: three and a half.

... with the small modification, take a little more, e.g. because of the insulation and a somewhat more comfortable parking space, let's say: three and a half.

I will try to negotiate that. However, I assume that the owner does not want to sell. I think that I have the best chance to negotiate a right of first refusal with the owner. There is even the option of a real right of first refusal, where the priority to purchase is entered in the land register.