Construction of a Single-Family House - Assessment of Our Financial Situation

We also don’t have any oddities with 2x disability insurance, 2x car insurance, 1x liability, 3x accident, 1x legal protection, and 1x household insurance, but that easily amounts to €400 per month.

Off topic You should check if all that is necessary. Legal protection? Is that really needed? For what? Especially disability and accident insurance, there is probably some double coverage. Annual payment saves money. Also, with (why isn’t the age entered anymore?) young age, your car insurance might be somewhat higher.

You are, of course, raising a very good point. The folks at the tax office also want to earn something from my rental income. Unfortunately, I am not an expert when it comes to taxes, quite the opposite. However, I would assume that the income is taxed at my or our tax rate. Which ultimately really raises the question of the meaningfulness of keeping the apartment.

Then my rental income leads to no higher income that could positively affect my financing.

The tax aspect definitely has to be examined if you decide to keep the condominium. Well, rather before you decide for or against it!

Thanks for your initial inputs.

Yes, the income is taxed at the personal tax rate. That would be €378 of the €900 monthly at the highest tax rate.
So about €300 remain per month according to your calculation.
However, costs for the apartment can also be claimed for tax purposes. So the tax burden will probably be lower.

And yes, it is forced savings. Depending on the location, people have to live somewhere. And in fact, NO ONE knows what will happen in 30 years. This applies to every form of investment.
Besides, you could offer the bank the second charge in the land register of your apartment. With €100k remaining debt, there should be something left.

If financially feasible, I consider it sensible to keep the apartment. To build wealth, you would otherwise have to invest elsewhere on the side.

And yes, it is forced savings. Depending on the situation, people need to live somewhere. And de facto NOBODY knows what will happen in 30 years. This applies to every form of investment.

Sorry, some things are already known. That was the case with the corona infection numbers, and is the same with population development. The principle of hope helps little here. The children who will start new families in 30 years have already been born. And you can compare the ratio of these birth cohorts to today's generation and their predecessors. There will inevitably be fewer..

Hmm, the birth rate has actually been rising again since the 2000s. It was higher in 2019 at 1.54 than, for example, in 1975 (old federal states). 2021 will certainly be one of the years with the highest birth rates in the surrounding decades. If you just sit at home and eventually finish all the Netflix series, cuddling is also quite nice again.

The children who will start new families in 30 years have already been born. And you can compare the ratio of these age groups to today's generation and their predecessors. There will inevitably be fewer..

This overlooks the fact that people do immigrate. I don't believe that this trend will completely collapse.
Besides, it is again a question of location and area. The "popular" (= thriving) areas will, I guess, not empty out in the future either.
900€ cold for 115m², however, does not sound like a top location in a prime area; that's rather quite average.

Otherwise, we have discussed a similar question recently. With an apartment and a single-family house, the burden at best is borderline. At least if the rental income does not go well. With only one apartment, it's basically digital: you have a tenant who pays, or not (in the worst case, you have a tenant who doesn't pay). The burden still remains, meaning the installment for house and apartment. It could well be that the bank doesn't cooperate in this case.

With something around 5500€ net, the household income is not exorbitantly high, so that one could pay 350k€ existing loans plus the house to be built, let's realistically say 500k€, minus 50k€ equity, in total 800k€ (!!!) out of petty cash.

I think this will only work here if the apartment is sold.

How long is the loan for the apartment still running?