About the fireplace: no, gas is unfortunately not possible. And yes, I want a fireplace because I don’t want to be dependent on a single heat source. Furthermore, I have "experience" with houses that have a fireplace.
This seemingly positive "experience," where a fireplace, as you currently plan it, especially with underfloor heating, would certainly interest many here (including me); also the apparently positive insights regarding the necessary distance to such a fireplace due to heat and perception of the fireplace would interest me. In the meantime, you have moved it around a few times, finally considered the necessity of the fireplace chimney on the upper floor, even though it just takes up a little space there again.
Where it is positioned now, you probably will never have it running. The dinner guest’s ice melts before he has tasted the first spoonful or the thigh is scorching, the bar mate needs sunscreen on his back. It still is TETRIS when you try to squeeze individual wishes into a predetermined space somehow.
Regarding the door to the garage: what is the alternative?
Simply leave it out, save money and enter through the front door, it’s only three steps, or pay and lose the interior room, which is already tight anyway.
Ground floor guest bathroom: it’s not just for guests. What could the extra space be used for?
I understand why no bigger bathroom if there is space. But your current problem shows exactly at such a point, because you apparently think you couldn’t do anything with the gained 2 sqm at the front since you are playing Tetris and want to immediately sensibly spend the gained space next door. But it doesn’t work that way. 2 more sqm in the living room would be great – but that would only be possible with an open-ended planning result, not like it is now!
With the fireplace on the outer wall: so in my case with this positioning the chimney could also run along the outside, correct?
That has to be examined in detail, we do it this way for various reasons but also for space and cost reasons. That all must fit together... windows on the upper floor, height, etc. But I rather think that the heat in the small rooms combined with underfloor heating will be difficult, but apparently you have contrary "experience" there. If the fireplace is only a "toy" for now and then and less for heating, then it can even be solved with a built-in, external, large gas bottle. Maybe not the first-class solution but the current one even less so.
Actual sofa or other furniture dimensions: not for the sofa,
Banggggg.... why not???? You have such a tight space down there and don’t want to get a shock after construction, so why these inaccuracies in such an important place? Sure, a beanbag always fits....
but I want the table, for example, in this size.
"I want" I already know from staircase, fireplace, etc., and of course you can do that in YOUR house. But this desire also has consequences, especially with your tight layout.
Afterwards I measured it, almost 4x4.5m. I perceive that as large.
18 sqm can be sufficient depending on taste but definitely is not "large," all of that depends on your wishes. Do the parents also have the straight staircase, this fireplace, a family with small children...?