Extension on an old building from 1965 with an additional storey

oh yes, does the building already have facade insulation? It will become mandatory. An example of many additional costs.

I would like to know if anyone has already carried out something similar. [...] If anyone has experience reports with a similar project,

Yes, - that would be, in my opinion, the most comparable project here, as far back as (four years) I can see.

Maybe there are also people here who can assess from the plans whether an extension here is even possible.

Yes and no. An enthusiast might dare, but a businessman would not do it. Despite the year of construction, your initial property is a post-war economical build, and your wish is more demanding. After all, you want to insert a whole upper floor between the ground floor and the attic, i.e., raise the attic by a whole floor. That will be at least sporty with the non-load-bearing wall under the ridge; I don’t see the construction carrying even a considerable upgrade of impact sound insulation – not to mention underfloor heating. Besides, with the extension you create walk-through rooms, where the property is conceptually already no longer attractive by today’s taste. Modern high school seniors roll their eyes “Dad is embarrassing” if you suggest passing the result of this conversion to them one day.

But if you see that you also want to move a (load-bearing/bracing) wall in the ground floor, it will become difficult

Even if probably such a wall in the future upper floor is meant, yes, you probably cannot remove it there. And I mean that emphatically “not at all” – not just “not with economically justifiable effort” – because: I do not see it substitutable here (except possibly by giving up the converted attic and switching to a trussed roof). An apartment of the same size for the grandparents elsewhere (and then less barrier-prone than here on the raised ground floor) will be financially more attractive than just the additional effort here for inserting the floor between. With “extension,” the project is actually euphemistically labeled. Look at the example linked above by to see how a properly dimensioned conversion of a slightly older, but technically more suitable property looks in practice. You would have to invest comparatively more money and more time here and would still get a less satisfying end result.

Hello everyone,
thank you very much for your assessments.

: Yes, the budget is a target budget that could certainly be increased by €100,000; for this project I imagined €200,000, after all, it is a renovation and not a new build. If the planning says it will be tight, then I will have to reconsider.
Yes, electricity and water also need to be done, although the ground floor including windows, electricity, water, and sewage was completely done two years ago, including the distributor in the basement, etc.

I do not want to move any load-bearing walls on the ground floor; the external wall is already slightly set inward. Or do you mean another one?
Only a door should go into the wall to the study, which hopefully will be possible.

No facade insulation has yet been installed. What do you mean by mandatory? If it is being renovated or if funding is applied for?

: Thank you for the link, I will take a closer look at it again this evening.
Yes, the base is certainly not the best; the renovation of the ground floor has already shown quite well what is possible/necessary. Honestly, the current upper floor will more or less be torn down and rebuilt together with the new upper floor.

Yes, I have to take a closer look at the issue of walkthrough rooms on the ground and upper floors. In the configuration living room-study, that is still acceptable, but as a bedroom behind the living room certainly not suitable for everyone.
After all, the idea would also be to make three apartments out of it in the future (if necessary).
The option of a separate apartment for the grandparents is rather unlikely; in that case, I would rather reorient myself.

Thanks again for your assessments so far,
I will keep you updated

At least the idea would also be to make three apartments out of it in the future (if the case arises).

Never. The tenants you would get for such apartments, you wouldn't want them even as a gift. Or is the location excellently close to the university?

Yes, you are right. One insight from this discussion is that I need to reconsider the classification or the cultivation, it certainly won't be the last change.

Subject to photos of the property, I think according to the plans that it will not be a bad property to make it nice. But major changes to turn a boat into an airplane will not be worth it (or possible). One should not only avoid comparing apples with pears, but also avoid turning them into pears.