Is renegotiation possible for a home loan?

Possibly yes, for example: The financing at 1.8% has been running for, for example, 9 years and is fixed for another 6 years, but interest rates are now at 6%. The bank asks: How about we refinance now for the entire remaining term at 2.5%. That is exactly your example. I would definitely consider whether you wouldn’t go along with the upward adjustment...

Regardless of everything else, I don’t understand your example. How long should the remaining term be? Longer than the 6 years, I assume? Also, the question arises why the bank would offer something like that? The bank also has to "secure" the business, which would not be very favorable in an interest rate environment of 6%. But that would get too financial-mathematical now. And overall you must not forget one thing either: the entire processing and associated administration... I can’t imagine that a bank sees any advantage there unless it is afraid that the whole thing is going downhill.

The banks and building societies are constantly trying to get rid of well-interesting investments like old building savings contracts, etc. and to talk customers into it...

yes exactly INVESTMENTS, their liabilities business. They don't want to worsen their assets business after all

Possibly yes, e.g. the financing at 1.8% has been running for, for example, 9 years and is fixed for another 6 years, but the interest rates are now at 6%. The bank asks: How about we refinance now for the entire remaining term at 2.5%? That is exactly your example. I would definitely consider whether you would not go along with the upward adjustment...

yes follow-up financing, but in the specific case the loan has only been running for 2 years.

One should differentiate.

So, a final answer: Thank you to everyone who participated constructively. The discussion was very insightful. There actually would have been some room for maneuver. An extension of the existing loan on the same terms was being considered. So from originally 15 to 20 years and longer at the same rate. However, we decided against it, among other reasons because by using other strategies, we tended to pay off the house earlier.

What does room for manoeuvre mean? An extension was offered to you on the same terms. The bank does not incur any interest loss from that. And an extension on those terms would probably still have been worthwhile for the bank. 1.8% for 15 or 20 years from today is not exactly exciting.

Worse still, due to the extension, the counter for the special termination right starts again at zero.