One-sided or double-sided fireplace

No, not on the glass, but definitely on a bench as planned by the OP. It was a bit misleading.

Something like that would also be suitable. There are enormously many possibilities.

Edit: with the installation situation of the OP, a custom-made bench seat is then necessary because of the corner in the wall?

Why a custom-made one? If it is the L-fireplace, the bench seat then simply goes right up to the chimney, the other side (so not in front of the glass) works the same way..

Here are two more pictures from the internet for illustration...

Bild 1 would be the one-sided fireplace, presumably the gray part is actually set a bit further forward. To the left of the gray part is the chimney, to the right of it I can imagine the built-in shelf like this...



Bild 2 would be the two-sided fireplace... presumably the fireplace part is set a bit further forward, since the L-glass must remain in front of the chimney, which you have to imagine here to the right of the fireplace part. In my case, the whole thing is mirrored. But this is how I imagine the shelf.

Let's leave out the conservatory for now... the living room must first serve as the basis for evaluation

From my point of view, that would mean: "Worst-case assumption that the extension would never come" equals "Result: heating so unnecessary that the fireplace would always stay off." Which brings us back to my classic, to use a thermally neutral HDTV fireplace ;-)