Evaluation and Improvement of the Financing Option

I would assume that the fixed interest period for both loans ends 10 years after the contract date, regardless of whether you now have a one to three year repayment-free start period with KfW.

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 it mathematically. Do you have any tips for that? I would like to see what interest rate will be shown after 10 years.

The interest rate remains the same during the fixed interest period. But since you are making repayments, after the first installment you already have the original debt minus the repayment portion. And thus you are paying interest on a smaller loan amount starting with the second installment.

Option 1 will only pay off if after 15 years the interest rate is higher than 8.4%...

Funny, I get 7.4%

Do you have Excel?
If yes,
then you make a table
Column A: Date (can also simply be a counter column, each row gets a new number (one month later) 1-300 or so)
Column B: Loan amount (beginning of the period), just enter in the first row, from row two onward = cell reference: *Column G of the previous row*
Column C: Annuity (the total amount you want to pay monthly)
Column D: Interest rate (the percentage (nominal) that was agreed upon)
Column E: Interest = Loan amount * Interest rate / 12
Column F: Repayment = Annuity - Interest (Column E)
Column G: Loan amount at the end of the period = Column A - Column F

From row two onward, you have to take Column A from Column G of the previous row (simply = and click on the row).

And then you drag down.

After 10 years (120 rows) you can play with the interest rate. Simply sum the interest, repayment, and annuity over the rows above, then you see how the total interest paid and repayment relate to the total sum of the paid installments.

Would KFW300 not be an option for you? The income should fit. The requirements are also not as high as one might think, we are currently building a solid basement with waterproof concrete. With the combination of facing brickwork, photovoltaic system, and heat pump, you can easily achieve the LCA.