Building savings contracts one-time interest rate changes, where is the catch?

We would do both. At least that was the initial idea. Prepayment of about 5000/year, and building savings of 250 per month. But I’ll calculate that again, I think you’re right. 250 per month would be another 3000 per year. If we put that additionally into prepayment, then only 1000 would be missing and then theoretically we would be done after 20 years.

We would do both. At least that was the initial idea. Special repayment of about 5000/year, and building savings 250 per month. But I will calculate it again, I think you are right. 250 per month would be another 3000 per year. If we put that additionally into special repayments, then only 1000 would be missing and theoretically we would be done after 20 years.

Then your remaining debt should be so low that you don't have to worry about it. In 20 years the money will also be worth significantly less, meaning a remaining debt of 200k is more like a good 100k today. That wouldn’t really scare me much.

but if I borrowed 500k, and only have a 10-year fixed interest period, and after 10 years still have 400k outstanding, what makes more sense then?

I don't want to leave it at that. Anyone who takes out such a financing will never be finished in their life. And in this case, that is really to be taken literally. The right advice in this case is not to secure the 10-year remaining debt of a 50-year term but to reduce the term. Everything else is financial kamikaze. Regardless of what interest rates or real estate prices do in the next 10 years.

what I have already seen ;)

but 500k with 10 years fixed interest rate and 2% repayment (at 2.50% interest) results in a calculated term of 32 years... so if you retire in 35 years, it fits on paper for now...

but sure, the optimal way is best to have 20 years fixed interest rate, and then arrange it so that after 20 years there is a manageable residual debt, which you can actually pay off with special repayments within the first 20 years.

but at the high ends (90% or more) 20 years fixed interest rate or more is so expensive that it is almost inevitable to rather take 10-15 years fixed interest rate + secure it additionally with a home savings plan.
in terms of total costs (incl. closing fees) it is considerably cheaper than taking 20-30 years fixed interest rate.

the charming part is then: 15 years bank, 15-17 years home savings bank, at fixed conditions, fixed rate over the entire term, and then you are debt-free, without interest rate change risk... and with special repayments you additionally shorten the term and thus save on interest...

what I have already seen so far ;)

Not everything that is financeable and financed should actually be financed. I think we agree on that ;) For a 400k building savings sum, you need about 150k saved. Let’s ignore the zero interest rates, that’s then a good 12,000€ per year or 1,000€ per month over 10 years. That would turn your 2% repayment into a 4.4% repayment and correspondingly significantly reduce the outstanding debt. Then you also don’t have to worry about the follow-up financing anymore. If you keep up the rate, you’ll be done after another 10 years even with sharply rising interest rates. And with that rate, a longer fixed interest period would of course also be unproblematically achievable. Sorry, but I still don’t see a concrete example where the building savings contract is seriously the better solution. Either you cannot afford the interest surcharge for a longer fixed interest period. But then you also don’t have money to invest large sums “on the side” in the building savings contract. Or you have the money, but then you can also agree on a corresponding rate with an appropriately long fixed interest period directly without any closing fees and without a negative interest rate differential business. The combination of low rate, low costs, and long interest security would be like money given away. And that is simply unrealistic.

I'm sorry, but I still don't see any concrete example where the building society savings plan is seriously the better solution.

With an expected inheritance/gift/sale, etc., meaning you know a lump sum will come in in x-y years that is too large for a special repayment. Here you can put the chunk directly into the building society savings plan. At least that's how we did it, but to be fair, I have to say that we didn't pay any closing costs.