What can we finance?

If I sold the house after 10 years at a value of 30,000€, I would have 239,000€ in hand. As a renter with 1% interest only the 167,000€. That makes a plus of 72,000€ for me.

That is, with respect, the mother of all naive calculations.

At the beginning, it was mainly about the saying that owner-occupied single-family homes are suitable as retirement provision. Arbitrarily narrowing the observation period to 10 years now is not useful. If I choose the right time window during a period of guaranteed value increases, I can achieve almost arbitrarily "good" (= good-looking) results.

Besides, the 1% is arbitrarily set too low. Anyone who deals a little with investing achieves >5% in the stock market over many years as an average, or acquires real estate for renting out (where others build up your capital stock with their cold rents). As you yourself have calculated, that is enough to even beat your house in the 10-year window. However, the compound interest effect really pays off only over longer periods, which is why, even if your house is paid off, you can hardly save up for it afterward if you want the same returns at retirement.

The correct way would be to calculate what costs an owned and paid-off house saves you in old age and to compare whether these savings are larger or smaller than the returns of other investment products that one would have built up with the same repayment and additional payments a house requires over the same period one lives there. That’s the only way.

If you don't take 10 years and 1% but 30 years and 5% (which I consider much more realistic), things look very bleak for the house very quickly...

Apart from the fact that you are solid anyway with a 15-year full repayment plan (did I understand that correctly?): The average person today rather builds with a 20-year repayment target or even more. That makes the consideration for the house even worse.

The reserve fund has to be taken out there, or would you pass it on to a potential buyer after 10 years?

If it has been used, of course. Otherwise, a buyer comes after 10 years and says the heating system needs to be replaced soon, and I will deduct that from the house price. Therefore, it must not be ignored.

Likewise, you can easily factor in an increase in value. Even Kommer, the biggest single-family house hater there is, sees that houses increase in value by one percent above inflation.

Realistically, you can rather estimate at least 400k after 10 years. In the greater Dresden area, house prices are currently rising by 8 percent p.a. according to the DTI index.

That might be true under current conditions, but you never know what will come. That is why my calculation was rather cautious.

With all due respect, this is the mother of all naive assumptions.

At the beginning, it was mainly about the saying that owner-occupied detached houses are suitable as retirement provision. Now arbitrarily narrowing the observation period to 10 years is not productive. If I choose the right time window during a period of guaranteed value increases, I can achieve almost arbitrarily "good" (= good-looking) results.

I can also do the calculation over 30 years and claim that stocks should easily yield over 10% if after 15 years I have no more installments, factor in rent increases and the increase in land value.
In my opinion, I have calculated very conservatively.

Apart from the fact that you are solid with 15-year full repayment (did I understand that correctly?), the average person nowadays builds with a 20-year repayment target or even more. It will look even worse in the analysis for the house.

The planned term with 2% initial repayment was intended to be much longer. Due to a larger salary increase, we have been able to fully utilize special repayments every year from the start.
It may be that this works in our favor.

I have also emphasized that the calculation only applies to us and can look completely different in other circumstances.

The value of a house actually never increases, it only ever decreases (wear and tear)..if anything goes up, it’s the land. Whether land prices will continue to rise as sharply as before is pure speculation. When interest rates are significantly higher again at some point (and they really should be)...what do you think will happen then? The circle of people who can finance 500..600k (and that’s the minimum required, since building costs certainly won’t decrease, that has never happened) at a few percent interest will shrink significantly. In addition, people with relatively high incomes prefer to build themselves rather than buy existing properties. So who is supposed to buy all these houses then? Then there is the massively aging population who will want to move out of the houses (because they’re too big). Overall, not so rosy...small houses in the best locations might be better...larger houses in not quite as good locations will quite certainly not gain in value. We have an AAA location (big city + large lake right outside the door) and still I’m sure that currently no one would pay the already invested 1.2 million for our house. I’d guess max. 900k...and even then the buyer pool here in the East is probably extremely small. If anything, this “group” builds themselves. I just hope never to have to sell, even though I would really like to build again.

The value of the substance also increases with inflation, more precisely with building costs. Keyword restoration costs. Masonry, base plate, etc. are also not subject to depreciation.

Plots in urban areas are becoming increasingly scarce, new construction is simply no longer possible in many places.

What no one will pay you for are, for example, your KNX hobbies. Even if you make a pink bathroom, you have to write that off completely in your calculations.

Furthermore, the 1% is arbitrarily chosen low. Anyone who deals with investing makes >5% at the stock market over many years on average, or acquires real estate for renting out (where others build up your capital stock with their cold rents).

Blablabla..
The performance of the MSCI World GTDR over the last 20 years was 4.87 percent per year before all taxes at the fund level, personal level, fund expense ratio, and before inflation.

The real return was therefore 1 - 1.5 percent at 100 percent in stocks (ETFs). You can do it, but it is risky.

With a house, the biggest part of the return comes from the saved rent. This form of return is less risky than government bonds or overnight money.

In the greater Dresden area, house prices are currently rising by 8 percent p.a. according to the DTI index.

Maybe at the moment, completely uncertain whether this will last in the long term. A relative was tempted in the early 2000s in Barcelona to excessively leverage his condominium and buy a second one for rental purposes. Banks financed almost everything back then. Reasoning: "Everything keeps getting more expensive/valuable anyway." Bad luck, even there the trees didn’t keep growing forever. He is still struggling to pay off the loans today. Mind you: the condominiums are in the historic city center of Barcelona and not on the outskirts of Dresden. What is currently happening in the German real estate market and, to some extent, again in the US market price-wise (even in the middle of nowhere) strongly resembles the events in the USA and Spain before 2007. In this respect, Steffen80 is absolutely right: as a long-term investment, stocks are much more sensible. Temporary setbacks are compensated much faster there than in the real estate market.