Floor plan of a single-family house with basement, 150 sqm, only single-story allowed

Well, I have a bit of the feeling that you don't really know yet what you truly want and need. But you want to start the final anyway, without clarifying that. That is not good, to put it mildly. The list of Musthaves and No-Gos must be fixed. Otherwise, there will only be disappointment and trouble.

Btw. Be careful with links to external sites. This can lead to the account being blocked.

Well, I have a bit of the feeling that you don’t yet really know what you actually want and need.

Yes, at our company this is called the "Requirements Capture Phase." And just like there, the customer (here me/us) doesn’t know what they want unless someone who knows their stuff nudges them in the right direction.
For us, as I said, it’s difficult because we have never lived in our own house and have mostly adapted to the apartment (where we have lived for 12 years). We don’t know that environment well enough. I also don’t see any way to figure it out without living in a house. The uncertainty of the future (what will the children need as they grow older?) remains.
I tried to answer all the questions that came up as honestly as possible (which I hope is noticeable).
We are probably also the type to say "next time I’ll do it differently," but at the current time I don’t have more information than I have shared.

In this case, it is really a pity that you are only here now. The needs in particular can be best discussed with other home builders. But better now before it's too late.

Dear , thank you for your open reply! With my partly exaggerated remarks I definitely do not want to annoy you, but rather nudge you to think about it as well.
The "problem" therefore seems to be rather that you have not yet found a joint needs plan. In this respect, I see it exactly as does, as equally fundamental and "dangerous" to already start with the final stage. Pressure and frustration during construction are almost always present, and when mutual disagreement is added, the situation can quickly overflow.
By nature, I am more gut-driven and always need to see things before me, so I examine details such as stairs, distances, heights, etc. closely in order to be able to decide for myself.
Of course, a good independent architect could guide you through the project and also give you the necessary security or act as a mediator between your ideas; a planner from a general contractor will often only implement this to a limited extent in your sense.
However, you cannot be taken away the true exploration of your needs. I understand that this is the first house, you are not alone with that, and even though we have gone through this several times, there are always things that you cannot foresee or imagine beforehand.
For example, we did not know where our favorite spots would be—now it is, for instance, our super comfortable dining room sofa, and we hardly use the living room because everyone often looks for their own corner in the house and does their own thing. You are a family with kids, and so it makes sense that everyone can have their own nice place for themselves.

We know ourselves too little for that. I also see no way to find that out without living in a house. The uncertainty of the future (what do the children need when they get older?) remains.

That will also change in the house, but simply creating a lot of space from the outset to allow everything to happen is not advisable. That was what was meant with the "broom closet." A nice tall cabinet, for example, in the dressing room or bathroom can accommodate all that and does not require an extra room with door, wall, electricity, and planning—or the broom closet becomes the utility room with washing machine, storage space, etc., then an extra room makes sense.

We are probably also the type "next time I'll do it differently,"

You are not alone with that either, and everyone here could, if self-critical enough, tell you about their own mistakes made during construction.
I really have the impression here that the basement was planned precisely because of this uncertainty, to be placeholders for all kinds of things. However, it is far too expensive for that and ultimately does not bring the desired overall success, because a basement is a basement and does not become a sunny living space. Maybe you should find a trusted person with a clear view and enough reason to at least find a clear plan for everyday things such as: actually required indoor storage for vacuum cleaner, broom, cleaning agents, step ladder, extra kitchen appliances, suitcases, and much more—and then add a buffer, but one that does not necessarily have to end in a possibly six-figure basement. Perhaps a nice, dry garden shed also meets many of these needs for much less money.

The second bathroom can be "moved" to the upper floor

I would not do that, because you have guests or strangers in the house who should not go upstairs, but an extra WC connected to the bathroom on the upper floor due to shared pipes could make sense with growing children.

It is difficult to know for the next twenty years. I know what I know, but don't know what I don't know.

That is the case for everyone, so a plan or structure can help to be as well prepared as possible for the unknown. You can create sufficiently large rooms and then discipline yourself to make do with that. More and fuller is always possible. Therefore, you could also base it on the current need for clothing, etc., plus a small buffer. But you will not have to have double the amount of clothes for yourself and your wife as you have now.

As you noticed, my wife likes to store things somewhere without bringing them to the basement.

If she does not change or want to change that, then she should plan this multipurpose room on the ground floor as she needs it. Then that is one of those fixed specifications for a planner to plan exactly such a room on the ground floor. And it is the same with your office… step by step, room by room, firmly define the needs and then maybe at least a few of the knots will resolve.
These must-haves such as multipurpose room, possibly small extra WC on the upper floor, your office really as an office, and a pull-out bed for rare spontaneous overnight guests, sufficiently producible seating area in the kitchen/dining area also for the often present extended "big family," tall cabinet in the bathroom/dressing room for vacuum cleaner, cleaning agents, bucket... I would write into my "specifications" little by little, and if an oriel bay window results from it, that is nice but not the other way around. The house must function for the inhabitants on the inside, so I would proceed like this and then see which options arise.

I also see no way to find that out without living in a house.

There is a reason for the saying: "The first house is built for the enemy, the second for the friend, and so on." But ultimately, this saying is not always applicable, because often the first house meets the needs, but these change. Also, some things are unimportant in a young family. Only when life becomes quieter and you have adapted to children, family, and no longer just the partnership, do you start to improve, rearrange, and then dissatisfaction with the house often sets in. But one must not forget that usually 10 years have passed by then.

I find a bay window "for practical reasons" generally difficult to imagine. I assume it is mostly built for aesthetic reasons.

A bay window is often built so that you get standing height in the middle of the roof surface for a central room in the attic. Or you capture daylight in the middle of the roof surface by placing windows (as can be seen in your design).

As you may have noticed, my wife likes to store things somewhere without taking them down to the basement.

I can completely understand wanting to store things where you need them. That is why many nowadays decide against a basement, because the unused space – except for technical rooms – would not be justified. Maybe you should include the basement more in the planning if you cannot do without it. For me, it would be unnecessary according to your needs. Although you want to accommodate a hobby or office down there, you must not forget that, due to the single-story design and gable roof, you still have nice space in the attic that could be developed. Lawnmower and outdoor gear are preferably kept on the ground floor, i.e., in a shed without having to use stairs for these items. You can get 2 or more rooms in the beautiful attic without having to install artificial light shafts on the house. With a floor area expansion on the ground floor, many things relax. But if you don’t want to do without the basement, then include it right away in the house planning and not like here: “doesn’t matter, you can just fit an office in there.” I see absolutely no reason to fuss over the development plan here. Why? Only if you absolutely want to build the city villa. To me, a gable roof house, even single-story, has far more advantages than a two-story one. Regarding your current design on page 1: The problem is the staircase. Because it is pushed forward, a bottleneck forms towards the back (SW). It does not allow for a dining area if there are doors still in the hallway. The hallway, in turn, is not sufficient for a proper cloakroom and an access to the pantry. You can twist and turn: it’s problematic. Especially upstairs, where sloping roofs reduce standing space. If you build 20 sqm larger and want a walk-in closet, this would ease the situation a bit. But this should not be an encouragement to develop new needs just so it fits. Most houses with a central bay window at the entrance have the stairs right at the front door. This relaxes the open living area, and the upstairs area is often used as a light catcher for the hallway. Everything has its pros and cons. Regarding the Nordic style: Nordic classic houses have little or no knee wall, often roof pitches over 42 degrees. Just a note. Tip: Read the floor plan discussions here in the forum. (“Long and narrow” you can skip). Take a look at the arguments and needs of the participants here. Sit down together on the weekend and write down your needs and then your room program. Ask yourself in which floor what can be well and practically accommodated. Not everything is feasible anyhow. On storing and tidying up: Don't plan small clutter corners everywhere, but rather a central cabinet area. Of course, it is “allowed” to keep a wardrobe for seasonal clothes in the office, but to use the room as a kind of airlock and big storage room, thereby depriving it of its purpose, is wrong. Then you should also name it accordingly so that you recognize what you are actually planning there. Because right now it looks as if no room is really something proper. And always mark the furniture yourself with a pencil. That is what the plans from the planner are for! There was something else, but I forgot.