Interior window sills - What depth or overhang?

Are there houses built by your [GU] nearby? One could take a look there to see if it's the same or if he mismeasured and is just trying to make excuses.

Oh, that is too complicated for me, I can't just run into a stranger's house.

Just spoke with GU.
As suspected, he doesn't understand the problem.
He has always installed it that way.
I told him I can't live with it.
If necessary, I'll just have to pay.

I'm just looking at my windowsill in the office.
You're right when you say that's how it's done when there are radiators underneath.
And when he says "we always do it like that," he means "with radiators."

He should think about it a bit, he definitely messed up himself, that you have underfloor heating.
Is the house otherwise equipped with radiators? Do you have an upgrade with underfloor heating? That also includes changing the windowsill!

I wouldn't tolerate that – and I've tolerated a lot!!!

No, it's all underfloor heating, he just means, he always does it that way, no one has complained yet.

Of course he says that

That is the problem with craftsmen. In our prefabricated house, it was determined during the selection process. We also took 3cm.

Well, we'll see, meeting tomorrow for other matters, let's see how we come to an agreement