Building a house at 21 years old .. too young?

Hello Daniel, I quite like it.
I assume that the bay window is on the south side.
The basement is probably not planned either.

I still have a few things to mention:
I can't judge whether the load-bearing walls are suitable.

If you make the bathroom upstairs smaller, then there will be space for a storage room for washing machine, dryer, etc.
A pass-through for dirty laundry from the bathroom is also very practical.

I would prefer to enter the bedroom through the walk-in closet and arrange the closet rows parallel north/south.

I like the WC with shower on the ground floor very much, but unfortunately, I see no space at all for a wardrobe, shoe cabinet, etc.

If the washing machine, dryer, etc. are upstairs, then you could take a strip from the technical room to the left of the stairs for a wardrobe. The small window could remain for the wardrobe and the technical room would get a suitably sized window on the east side.

I would make the pantry under the stairs with a sliding door.
I am generally enthusiastic about sliding doors.
The distance between the stove, sink, and peninsula in the kitchen seems too large to me.

If your girlfriend also wants to build at a relatively young age, then go through it together.
My future daughter-in-law said that building the house with a lot of personal effort (fortunately still without a child) strengthened the relationship even more, although it is sometimes said that building a house can also strain the relationship.

I am totally enthusiastic about sliding doors.

"Because they are so easy to operate?" Irony off! They are absolutely bad to handle. Opening the door with your elbow, foot, or knee is not possible.

It's only 140m2, right? For that, I think it looks good. Much worse floor plans have already been shown and implemented here.

Yes, I don't think it's that bad either.. Not exactly optimal, but there have definitely been much worse floor plans. You can tell that you are still younger and place value on things that you might later regret because many things are not practical but only "look nice." We are also young but have different priorities for this.

- Since you "only" have 140m² and living/dining/kitchen is "only" measured at 40m², I believe it will feel very cramped. The table in the middle of the room, quite close to the window (it's only a small table)... it will be pretty tight there. The seating at the kitchen, phew, I don't know if that's needed and if it won't be "too tight" there as well. The narrow corridor from the hallway into the living area does not really feel open or welcoming but rather quite confined. I somehow miss something in that living area... Everything feels pretty squeezed together to me. I also find the pantry with sliding door pretty annoying, I don't think you'll be happy with that. It's not the biggest kitchen either. I can imagine (since I know how young people think) that the kitchen will be way too small for you. I would rather do without the seating area and plan everything differently. The bay window itself isn't really great either. It pushes everything further out in the area of the bay window and you have much more space, similar costs, and a lot more room.

- Standing space in the living/dining area is hardly given. You barely can put anything on the small narrow "bay window walls." Any space is kind of "blocked." I really don't think it is well planned.

- Office/guest room is pretty small anyway, narrow niche in the corner behind the door... No idea, it doesn't really appeal to me.

- Do you really need a shower downstairs? I would rather skip it and create more space elsewhere. Better plan a nice wardrobe in the hallway, which is completely missing!

- 5m² hallway? Holy moly, that's going to be very, very narrow, believe me. There is no space at all, you can't even properly install a shoe cabinet, a wardrobe, or even a mirror that actually serves you. Much more needs to be done there. The straight staircase also takes up an incredibly unnecessary amount of space, I wouldn't do that. Make a spiral staircase, that way you save space in the hallway.

- Utility room.. Yeah, sure, you need it anyway, that works since you don't have a basement.

- Tiny children's rooms but a bedroom with a walk-in closet. Think carefully about that. Do you really need it? What do you really value?

- If the shower downstairs is used, I wouldn't make the bathroom that big. If the shower downstairs is not used, then I would leave it as it is, that's fine.

All in all, it is not so easy to implement if you like my suggestions. Then it really needs to be completely replanned. But I claim that the floor plan for 140m² (which is really not that little) is not well done.

The usual points of criticism:

A dressing room for prisoners is not necessarily good, the children's room is probably on the shady side, but a dressing room faces south (assuming the terrace is on the south side) - but you need the sun there the least.

The bay window costs money and is just about chic! All alternative villas have it. But usually it’s a useless money pit. I would rather enlarge the footprint around the two side sections that are currently "cut out." In terms of price, it probably doesn’t make much difference overall, but the few extra square meters would be more important to me than the bay window, which is just trendy right now. Move away from what is currently so hip and in and think more about what makes sense.

I would put both children's rooms on the south side on the upper floor, since the bay window is dropped, the bathroom can be slightly enlarged (or one could consider partitioning off a small utility room for the washing machine and dryer) and the bedroom moved to where the northernmost child’s room is now. The room will also be somewhat enlarged toward the top of the plan; you first pass through a dressing room or wardrobe area and then into the bedroom (if the stairs should remain as they are).

The kitchen as it is in the plan is an ergonomic disaster, it can be done better and you have the space for it. The best workflow is always a kitchen with two parallel rows. For enough tips on this, I always recommend the K U E C H E N forum (sorry, I have to write it like this, otherwise it gets changed in AI forums).

Overall: maybe also consider a variant with a different staircase shape, because a straight staircase is nice, but it also costs quite a few sqm - with 140 sqm, you might gain a few sqm through a different stair design that could be used more beneficially elsewhere.

For example, for a wardrobe - you definitely lack space there. At the latest when the first child comes. Where is a stroller supposed to be parked? Should it stay in front of the door? As it is now, it will even be quite tight to shove it into the guest room. I also see no place for the coats/jackets/shoes/gym bags etc. of a family of four at all. You really have to work on that again!

Man, I could have saved myself writing my text, it is identical