Preliminary draft single-family house 12.4m x 9m, 2 stories, 22° roof pitch.

Tox is right.
Basically, the design looks neat at the bottom, a bit cramped at the top, but good. It is only after looking several times that the drawbacks become apparent.
Basically, I would switch the bedroom with the dressing room.
BUT: are you aware that you are claiming 40! sqm upstairs just for yourselves? I know: it has happened for you, or you want a wellness bathroom and a dressing room...
A bathroom for 2 people doesn't have to be 14 sqm, you have to earn the money first to furnish it as well. 10 sqm is more than enough. You can also adjust the size of a bedroom if a dressing room is built. Regarding the children's bathroom, I see the drainage of the toilet as problematic!
All in all, I would assign the open sqm upstairs to a storage room, look at the ground floor to see how the path from the kitchen to the exit can be planned more directly, or the wardrobe placed closer to the door to the garage.

Hello,

Give the supplier and the heating installers a 12m2 room and they will pretty much need all of it.

This is a generalized statement that may be true for your first house; by no means applicable to all current construction projects! As the builder, you have a considerable degree of say – one, about where the technical room or utility room should be positioned, and two, which walls can be used for the necessary connections.

But in the new house they had to manage with not even 4m2 and look – that worked too. Therefore, I would always keep this room as small as possible/necessary.

"As small as possible" describes a snapshot and is short-sighted; the correct statement would be "as large as sensibly necessary."

On the one hand, installers should at least be able to move somewhat in the room – otherwise it may become expensive for the owner later if device "A" has to be removed first to service "B." On the other hand, technologies are constantly changing; for example, if you currently decide on a compact unit (heat pump with hygienic tank), the space requirement is less than it might be in later years with separation into boiler and tank. It is also quite "simple" to consider – if you stick with one chosen system – that in year "X" you might want to install technology from another supplier. It could very well happen that this technology requires an additional buffer tank; this is, for example, the case with an Austrian supplier of heat pump technology. And then you’re in trouble ;)

A technical/utility room – i.e., a room without the requirement to store canned goods or whatever – should not be planned under 8.00 sqm. I also admittedly do not understand the idea that a larger technical/utility room could be a problem….

Rhenish greetings

On the one hand, installers should at least be able to move around the room somewhat

No problem with us. They even worked in there as a pair (placing the buffer tank, etc.)

maybe in later years with separation of boiler and tank.

We already have a 350l buffer tank and next to it hangs the indoor unit of the heat pump.

when device "A" has to be removed first in order to service "B"

We can reach all devices and connections if something should happen. Nothing has to be removed beforehand. All equipment and connections were arranged in an L-shape in the room, so the technology is only on 2 sides of the room. That was already taken into account.

That is a generalized statement which may apply to your first house;

However, I now know many builders who complain about exactly the same problem. Partly large rooms (12m2+), but if the washing machine and dryer are in there it’s simply full. No space to hang laundry or an extra cabinet for toilet paper, etc. So it’s not just me. ;-)

As a builder, you have a considerable amount of say

Well, the guys from water, electricity and Kabel D already told us where to put their stuff. There was no big say in that. Neither in the old house nor the new one.

I admit I also don’t understand the idea that a bigger HAR should be a problem ....

Space-wise, we couldn’t fit more without making the house bigger. We already had to make 2 rooms (office and utility room) smaller to even get our own HAR.

So I don't find the planning completely disastrous, but it's not exactly very well done either. On the ground floor, it's hard to judge without measurements and furnishing. Where is the 4-person wardrobe supposed to go? Behind the door is not very optimal. Is there no utility room? Is that included with the pantry? I keep wondering whether the moisture from the laundry might actually harm the pantry supplies...? How is the staircase area supposed to look/be designed? Looking at a staircase from behind is not exactly inviting (visually).

Upstairs: How is the bed supposed to be placed? Does it even fit with this corner? If anything, I would design the dressing room open - meaning door and back wall removed. In general, I am not a fan of large master bathrooms and small children's bathrooms. The children/teenagers also need to bathe. Then the master bathroom doesn't help except that everyone has to walk through the bedroom. Preferably a shower bathroom for the parents and a large family bathroom for everyone!

If possible, also post/describe the site plan and surroundings. You plan a house with property and garden.

Edit: Oh yes. What I still miss is the connection to the outside from the living room. With the corner window in the dining area, you will hardly be able to look into the garden from behind. Or what is the plan for this?

I am not completely happy with the draft yet.
A lot of storage space is lost due to the three doors in the pantry, so the room cannot be used optimally.
For a craftsman to get into your technical room, they first have to go through the living/dining room, then through the kitchen, and also through the pantry. Personally, that would bother me (a stranger walking through half of my house in their street shoes).

The kitchen is also too far from the entrance for my taste, since I don't necessarily bring groceries through the pantry entrance every time but sometimes through the main entrance, the carrying distance would be too far for me.

The upper floor feels somewhat convoluted overall, and if you build without a basement, I would still find a storage room on the upper floor important.

So I don't find the planning completely disastrous,

In case it came across wrong: I find completely different things disastrous, just not this draft. But with the dimensions, you can do better :)

Where is the 4-person wardrobe supposed to go? Behind the door is not very optimal.

I find this wall behind the entrance door acceptable for a wall unit ;)