Is purchasing a house and complete renovation affordable?

And on the topic of "old age". One does not automatically become frail at 70 and stop leaving the living room. Those who are in their mid-30s today have a good chance of exceeding 90, spending most of it in good health. Of course, at 75, one takes a few more breaks and maybe trims the hedge over 2 days. But that doesn't matter, after all, you also have a pension 7 days a week and don't have to fit everything into one Saturday like a 45-year-old. And if you don't want to do the work yourself, there is always the option of helping teenagers from the neighborhood with a small side job. With a low-maintenance garden, that's not a money pit either.

Your optimism is all well and good. But only very few of those currently 30 years old have a good chance of reaching 90. You are spreading false facts here based on feeling. The average life expectancy for men/women is still around 80 years even today. And from retirement onwards, for some unfortunately, it goes downhill fast. At 70 with a new hip, it's harder to mow the lawn and clean the house. By 70 at the latest, life in the house becomes a burden for many. Of course, it's clear that people don't want to accept that yet. But by 80, almost everyone belongs in a barrier-free apartment and no longer in a house if they want to live happily and carefree.

Well, average also means that there are plenty of people who live longer than this average value. And your statements about 70 or 80 just aren’t generally true. At least not when I look around here at those aged 65 to 90.

Life expectancy means that half will live longer. But that is only part of the story. A man who reaches 70 alive already has a life expectancy of about 84 years, because he did not die of cancer at 60 or in a traffic accident at 45. Women live a little longer. Life expectancy is also largely correlated with social status: work does not physically wear you down, there is less smoking, and people generally pay more attention to their health... When you add it all up, the life expectancy of the typical homeowner increases significantly. And then there are "good chances" (intentionally no number given). In this respect, it makes no sense to plan "for old age" and commit to an apartment from the start. Life is not happy and carefree when you have neighbors above, below, and next door and only a small balcony. When you have to go to the park (if there is even one) just to sit under a tree...

The OP is 28.
And here scenarios for life in almost half a century are being designed.
The question is, as also the title of the thread suggests, whether financing is feasible.
In my eyes, from my experience, now in my early 40s, with a family, two children, and financings sometimes with only one income, THAT would be too risky for me.
And we got into the really big financings when we already had one child and had been married for seven years and could really rely on each other.

Our plots are still around 650sqm, so that's still okay. I wouldn't want to build a single-family house on 350sqm either, as that leaves only a 3m clearance strip. 350sqm is OK for a semi-detached house.

And on the topic of "in old age." You don't automatically become frail at 70 and stop leaving the living room. People who are in their mid-30s today have a good chance of living past 90, most of it in good health. Of course, at 75 you take more breaks and might trim the hedge over two days. But it doesn't matter, after all, you have retirement seven days a week and don't have to fit everything into one Saturday like a 45-year-old. And if you don't want to do the work yourself, there's always the option to help teenagers from the neighborhood with a small side job. With a low-maintenance garden, that's not a money pit either.

Our neighbor across the street is over 80 and can be found in the garden almost daily, which is truly the best-maintained in the whole street. Only the lawn mowing is done by the neighbor.

You are absolutely right, but the question is what the alternative is. New construction (also condominiums) in the area certainly not, buying another existing property neither. That leaves renting, and Karlsruhe is not a cheap place for that either. And after all, this is a genuine major renovation and not just a bit of sprucing up.

If the value of the property after the renovation should not be at least equal to the investment, then I wouldn’t do it. Separation, career change, health problems with inability to work (Corona has made this very clear) and having to sell for significantly less than the investment? The dream of owning a home is currently not worth that to me.