Enlarge living room / extend concrete ceiling?

Roughly speaking, you can expect around 50k for about 10 sqm of additional living space. Maybe it will only be 30, or you might end up at 70. There is simply not enough information for that.

I hope I have answered everything.

No, as I said: complete floor plans including the basement and the floor(s) above that add additional load are indeed significant factors in serious considerations.

The rooms above and below the living room are laid out exactly like the living room.

That is too vague, and the window alteration is not a trivial matter.

Am I the only one who doesn't have plans???

Short version: ridiculous idea.

Well, plans gone because they are embedded on an external site and not uploaded here.

Hello,

unfortunately, the first post of mine can no longer be edited, I wanted to reinsert the plan.

I have modified the plans again and adapted them to the conditions. The construction execution does not match the plans 100%, hence the adjustments.
Here is the plan of the ground floor again:
[ATTACH alt="grundrisseg1.png" type="full"]39754[/ATTACH]

Here is how I imagine it in the side view:
[ATTACH alt="seitenansicht.png" type="full"]39755[/ATTACH]
Originally, it was planned to design the terrace as a veranda. However, this was not done, but it was provided for in the plans. That means the roof was basically just left out. Here is an example picture of how the veranda should have looked:
[MEDIA=instagram]B4cG5-OI-A9[/MEDIA]
That is, the roof of the veranda would have been placed on the concrete parapet of the balcony on the first floor. I would not have only made the veranda roof over the terrace (I am in favor of a veranda anyway) but would have extended it along the entire side of the house and also around the corner to the garage. This way, the extension of the living room would also be covered.