Follow-up financing with a building savings plan?

To be honest, I cannot quite follow your calculation. But here are a few general notes as a basis for your decision:

1. The costs of a loan are practically always significantly higher than the gains from any secure, fixed savings plan starting at the same time. Otherwise, a bank would not be able to operate. Perhaps under very specific circumstances an exception can be found, but you can fully trust the skills of mathematicians at banks (and also insurance companies!) in this regard.

2. Your special repayment therefore has an immediate and direct effect on your current interest costs. The interest portion of your installments becomes smaller immediately, the repayment portion larger. And the repayment increases exponentially over the term of the loan! €150 of repaid debt is significantly more profitable for you than €150 of interest-bearing money in the savings.

ok, another idea:

Before building the house, I had already paid into a Riester contract for a few years and used the balance as equity. I would now continue Riester with the goal of withdrawing the balance again after the fixed interest period ends.

My calculation: Starting next year, I will have two children. I would have the allowance transferred to me so that I would have to pay less personal contributions.

Example: 4% of gross income = €1,750.00 - allowance €154 - child allowances 2*€300 = €996.00 personal contribution

Projected over 15 years: €1,750.00 * 15 years = €26,250.00 total balance with only €14,940.00 personal contributions made.

I would withdraw the total capital as Wohnriester after the fixed interest period ends, so that the follow-up financing amount would be significantly lower.

Have I overlooked any fundamental things in the calculation? Where is the catch?

Financing costs, financing costs. The fact is that the interest rate level is currently historically low. One can speculate whether it will be similar in a few years when the follow-up financing is due, but one can also secure the low interest rates now through a [Bausparvertrag]. To me, that sounds sensible, it makes many things plannable and takes away some of the worries about whether and how the follow-up financing can be managed in the future.

It's just a little bit of poker with the home savings contracts. You have additional fixed costs like fees and an additional monthly burden that slows down the repayment of the loan (the home savings contract needs to be fully paid on time - and ready for allocation - I know that my parents, despite paying in as planned, waited 2 years for the allocation in the end, and the interim financing caused significant costs...). At the same time, you earn hardly any interest on the saved amount, but you pay interest on your loan every year. With compound interest, these effects initially lead to additional costs of the home savings contract amounting to a frequently five-digit sum over 10 or 15 years. You then have to make up for this again with a "good interest rate." You should also always look at the remaining amount for which you will need a follow-up loan after a period of, for example, 15 years. Most of the time, this is no longer high enough to worry about the interest rate.