Modernizing old buildings - planning, project design, construction supervision

Last but not least, one should also think about the partly non-existent or almost ineffective basement waterproofing in old buildings, which is practically impossible to access anymore.

I always wonder when I watch shows like "Bargain Houses" how people can be so naive and expect to bring the house back to a good (professional) standard with almost zero budget. Just yesterday, there were again "specialists" featured who bought a building for €40,000 and wanted to renovate it, although even the basic structure didn’t look good and basically all important parts need to be replaced.

And especially gutting is one of those things where you often have to expect surprises.

PS: It's good that someone addresses this because way too many people are naive and think they can save a lot of money.

And doing work yourself is always an issue: if something is done unprofessionally (wiring, heating, etc.), it can quickly become very expensive or dangerous if you don’t know what you’re doing. And even the great advisors at the hardware store aren’t always as good as they seem to a layman (especially since they can’t assess the local conditions).

Best regards

Not least, one should also think about the partially non-existent or nearly ineffective basement waterproofing in old buildings, which is almost impossible to access anymore.

Actually, one can straighten out quite a bit, provided it concerns vertical barriers, albeit with corresponding additional effort. However, the situation is entirely different when it comes to deficiencies regarding horizontal barriers.

....I always wonder when I watch shows like "Bargain Houses" how people can be so naive and expect to professionally restore a house to good condition with almost zero budget.

That is correct; unfortunately, "naivety" is the source of profit for some!
I recall a recent RTL feature about the "Bauretter" in connection with the IBG insolvency. Certainly, the builders were structurally helped in the specific case.
However, presenting an AWP for a presumably KfW 70 building as an "energy-efficient solution" lacks any foundation. Regarding the system technology, Kosmalla and Reibold-Rolinger are absolute amateurs, who should rather be reduced to superficial aspects (advertising effect)!
The excessive consumption costs will probably hit these builders harder than they would like.
In this respect, such features have more of an "entertainment value" than a generally professional, reliable presence.

...And also, do-it-yourself work is always a tricky matter: if something is done unprofessionally (wiring, heating, etc.), it can become very expensive or dangerous very quickly if you do not know what you are doing.

Completely correct, especially when there is no professional planning whatsoever!
Do-it-yourself work is helpful to relieve one’s own budget, but only if targeted and following appropriate planning!
One should factor in a time multiplier of 3 to 5 for DIY work!

Regards