Of course, that is really crap! But you have to evaluate it neutrally! Who did what and who didn’t. More on that below…
The GU is not your friend but an opponent!
He is neither a friend, nor is he an enemy, because he is a business partner chosen by you.
can be a fair and cooperative business,
It should be under normal circumstances. “can” rather implies unlikeliness.
If he already dares to bring up such things, I don’t even want to know what else he has saved on, changed, messed up, which you don’t notice later. Laying of underfloor heating, electrical wiring, etc. just as examples.
… Who did what and who didn’t?! The GU really messed up! But I don’t read that he concealed or covered anything up. Mistakes are unpleasant, but they can happen. “And then the building application is changed and the dormer is approved again.”
And what happened in between? What did the client do? _We do not know any of that_
We readers can assume that he silently tolerated it. Probably even accepted it.
For the construction company, it's probably "settled" with the re-approval.
Well, where do you stand in this game as the client? Either you speak up in time or stay silent forever. It reads as if none of this was uncovered just yesterday.
first stop everything further.
For what reason, if the client has no objection to the GU?
Now some inconsistencies have emerged, which are not really addressed.
Well, it’s your matter to address them. Your house, your money. Your order!
Do I really have to simply accept this “as is”?
No, but by remaining silent and doing nothing, you forfeit your rights. Surely the fine print in every contract requires that you verify the delivery, work, and service either immediately or within 14 days, and report complaints or defects within that period. That’s the law, as sorry as I am. Go to a lawyer, but I think even he can only advise negotiating a settlement.