Floor plan evaluation - saving square meters single-family house

The wall thickness outside is 42.5... The lines are a bit too far but I understand the principle. I also think that even if it goes to 11 or 11.2 x 8.7 +-0.2, it doesn't cause much damage.

Then the fireplace takes up even more space in the living room, right?

I would first think about the entrance/hallway, it is not inviting at all. 6-7 meters long and not particularly bright. Access to the living area only through the kitchen. The seemingly 20 sqm of window area in the open space doesn’t help.

That's true. You can only enter the living area through the kitchen. You don't always want that, especially not with guests. A double door or sliding door that leads directly into the living room next to the stairs would be an option. If it is glazed, more light will also fall into the hallway.

I think you have very little storage space, especially if possibly a second child is coming (I am assuming this now, since there are two children's rooms upstairs)... I would remove the door between the bedroom and the dressing room upstairs and extend the wall of the dressing room straight through (see picture). The dressing room can then be used as a storage room. By removing the door, you gain storage space. I don’t know how many clothes you have, but the wardrobe currently drawn in the bedroom should already be quite large. The blue lines are only roughly drawn, but I also think that you can start here and shorten the house. You can easily take a few square meters from the bathroom. Maybe also avoid a T-solution, as they usually take up more space.

Regarding the kitchen: freestanding islands take up even more space than the "U" (or provide exactly the same amount but then less workspace). As an alternative, maybe a tall cabinet wall with a "peninsula"? If I were you, I would focus intensively on kitchen planning right away. Think about how much storage space you need. What do you want a tall cabinet for or what should be elevated? You can fit a lot into a pantry cupboard. Then the pantry can be addressed as part of the "downsizing." The access to the pantry currently takes away a whole tall cabinet, which could be used more efficiently here.

But then the fireplace is completely out, right? Currently, it is 3.2m up to the fireplace and the entire wall is 4.1m

If a door is supposed to go in there as well?

Edit:

Bedroom: we actually don’t want a wardrobe at all. Currently, we have a total of 5m of wardrobes. We don’t know why it was planned even though there is a dressing room...

Bathroom: the T was a wish of ours and should stay if possible...

Child 2: is currently used as an office since we don’t know yet, we’re no longer in our mid-20s anyway ^^

Hallway: we thought the same about the corridor but really couldn’t come up with a solution

Fireplace: Fireplace is nice to have... We would only have to pay for the flue, so we thought why not... if it really doesn’t work, we will probably leave it out

Yep. The door would be at that point. Can't the fireplace be located somewhere else? Is it a must?