Old building renovation (1966): Two-family house - floor plan sketch

No, that still has to be built! Good idea!

In total, we now have 285 sqm of living space. Yes, it is a lot, but for the planned 5 residents (luxurious) it is okay, isn't it?

However, there are totals that are rarely achieved simultaneously: three children and two full-time earners, for example.

The stairwell will probably have to be remodeled as well, right? It’s definitely the typical stairwell with terrazzo and steel railing now, but not something you’d want in a living area, is it?

That will also be a space eater.

To be honest, I don’t know if I would want to live like that... (well, actually, I do know: rather not). The house is designed to have one residential unit per floor. Transforming that into a cozy overall concept is not so easy. A plan alone is not enough. Such a stairwell has to be turned into a living zone. I, for one, wouldn’t always want to feel like I’m walking through a public stairwell into my bedroom.

I for one would not always want to feel like I’m walking through a public stairwell to get to my bedroom.

Hehe, I know that, but the other way around: a former landlady of mine once decorated the multi-family stairwell so that it felt like you were walking through her living room. Unfortunately, taste was not her strong suit :-(

The stairwell wouldn't discourage me. You do have to do something, but it's still manageable. I would be more afraid of the heating costs.

Additional costs have already been a topic here...

I just see it this way: if I want to turn a house, which is old and designed for multiple parties, into a cozy single-family home that meets today's standards, then I have to invest a lot of money. And with nearly 300 sqm, that's simply more euros than with 150 sqm. And at some point, I would definitely ask myself whether it still pays off for me.
How is the basement? In the house I lived in (about the same year built; you remember: similar bathroom solution), the basement was soaking wet. It would have needed drainage and drying out. We had the pleasure of living on the ground floor. Which gave us a nice garden, but also always freezing cold feet, because the floor slab between the basement and ground floor was unfortunately not insulated like we're used to today or where the basement is also in the warm area. And when it got damp down there, it rose up the walls to us. We used 4,000 liters of heating oil in one year. For a 70 sqm apartment, mind you! Plus a Swedish stove in the living room. Cold feet were still normal.
The same applies upwards if the attic is planned as a cold area. If it was previously inhabited, I could imagine that there is no significant thermal insulation there either -> so that also needs to be done.

It's hard to judge how it is here from this distance, but you notice, I'm skeptical...
Does it pay off? Especially if the additional costs are so high? And obviously there are already restrictions. In my eyes, a facade renovation would be pretty much the first thing I'd do. Then bring the heating system up to the latest standard, also the electrical system and possibly the installation. And that already costs a lot of money, and nothing inside has been nicely done yet, that's still on top. But then the additional costs would probably have been trimmed down to a normal level first.
We also have people in the extended family who own an old house. Renovated with really a lot of love and commitment over the years. They wanted to sell it now, energy efficiency class F, and were surprised that they couldn't get rid of it.
Instead of renovating sensibly (i.e. insulating, heating, etc.), another old hut was bought and "renovated" (= painted, made pretty, energy efficiency class F again here too).
Well…