Evaluation and Improvement of the Financing Option

Correct me if I'm wrong, but with a fixed-term deposit account, you cannot set up a savings plan and must fix a one-time amount.


Why does the interest rate change over the course of the term?

Sorry, I meant a daily allowance account. There you can set up a standing order from the checking account.

The (overall) interest rate changes because the three components have different repayment rates. Therefore, during the term, the ratio of the interest paid to the borrowed capital changes.

Option 1 will only be worthwhile if after 15 years the interest rate exceeds 8.4%...

Could you tell me how this calculation works?

Is a KFW300 not an option for you? The income should be sufficient. The requirements are also not as high as one might think, we are currently building extensively with WU-basement. With the combination of facing brickwork, photovoltaic system and heat pump, you can easily meet the LCA.

Option 1 will only be profitable if, after 15 years, the interest rate exceeds 8.4%...

Funny, I come to 7.4%.
Though the assumptions after 10 years are interesting. I assume you continued to expect that the KfW loan after 10 years is paid off from parallel saved assets.
If that is basically possible, then there might be something to it, but the original poster should get offers right from the start based on the possible repayment amount, because with increasing repayment rates, often more favorable conditions are possible. Then a full repayment option might also be considered.

Because the problem remains:
after 10 years there is still a remaining debt of 42,000 EUR. For that, you won't get an annuity mortgage loan anymore. Maybe not even a variable one (too much effort for the bank). So you might have to take out a consumer loan to pay it off (high surcharge) or save alongside.

Set the annuity loan to a 10-year fixed interest period and consolidate the remaining debts, then do a follow-up financing.
-> Here, there is about a 0.2% interest advantage compared to 15 years. However, I see difficulties in having the terms of the annuity loan and the KfW loan end simultaneously (keyword: interest-only start years). What are your experiences regarding this?

Precisely because of the above-mentioned difficulty with the 10-year KfW loan, this option might not be so bad. On the one hand, you expose yourself to a higher interest rate change risk, but on the other hand, you can limit that by, for example, arranging a forward or something similar in time. Or simply repay quickly.
I don't fully understand your fear of terms with 2x 10 years. I would assume that the fixed interest period of both loans ends 10 years after contract signing, regardless of whether you have one to three interest-only start years with the KfW loan. But it's best to check with the bank and study the contracts (drafts).

Could you tell me how this calculation works?

The Excels from present it well. You just have to recalculate the interest and repayment share for each annuity and subtract the repayment from the remaining loan amount.
And then you can set up two variants side by side and play around with the interest rates accordingly.

For option two, you just have to list each component individually and then sum the absolute values in a table.

Where the assumptions after the 10 years are interesting. I assume you still expect that the KfW will be replaced after 10 years with assets saved in parallel.

Correct. I assume that the €41,328 will be saved in 10 years to repay the KfW124, and as the OP writes in #12, this would probably be possible:
"Putting aside a savings amount would tend to be possible, but you often read that one does not make special repayments anyway."
(sorry, I’m struggling with the multi-quote)
That is something you have to know and want yourself if the calculation is to work out.

The Excels from already illustrate it well. You just have to recalculate the interest and repayment portion for each annuity and subtract the repayment from the remaining loan amount.
And then you can create two variants side by side and play around with the interest rates.
With variant two, you have to list each component separately and then sum up the absolute values in a table.

Exactly. Since uploading Excel files and PDFs is not allowed, I recommend calculating it yourself once in Excel.

I somehow don't understand that. But the interest rate stays the same, doesn't it? I made a table, but I don't know how to implement that mathematically. Do you have any tips? I would like to see what interest rate is there after 10 years.