Floor plan corner bungalow 150m² with expansion possibilities

As marked, the terrace would be in the boundary area. That is nonsense and who would want to sit right next to the neighbor's fence?

I have to stake it out on the property in real life beforehand anyway, if there is not enough space I can also move the house further east. It would just be a pity if the trees at the edge would have to be removed because of that, as we actually want to preserve them.

I also find it all too convoluted and the hallway too dark. Besides, I am missing the access to the north garden. I recall your statement at the beginning and the great picture:

Our “beautiful” garden we definitely want to orient to the north towards the forest.

Here’s an idea for optimization with a large lift-and-slide door to the north in the living room, because you have the great view there and also a covered terrace that will get the evening sun from the west. I would make one terrace to the south and one to the north. And since you only have privacy in the south if the front door isn’t right next to it, I would absolutely place the entrance in the east. Also, a straight hallway with light through generous glazing in the front door and a glass door to the open-plan area. Additionally, a proper staircase to the upper floor in case kids come along and it needs to be converted. Turn the guest WC directly into one with a shower, for kids or visitors. I don’t know about the idea that the main bathroom must be accessible to guests and really don’t think it’s necessary. On the contrary.

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It would just be a shame if the tree stock on the edge had to be removed because of that, we actually want to preserve it.

Such a thing…

The connections are not yet clarified, what problems could there be? We theoretically have two connection options due to the two building plots.

… and such things belong in the rough initial planning. You have the plot, it is sketched in detail with the surroundings, then you can assign rooms according to orientation. And then eventually a plan comes out of it.

Children not planned for now, earliest in 5 years

But you plan something like that for your house if family planning is basically on the agenda and you look at that more positively than negatively: I would plan a house (at least) for the next 20 years. Especially at your age, consider options that might be possible. Where are the children supposed to live? You can't just convert the offices if you need them?! I apparently added up the hallway square meters yesterday. That would be too much for me personally. If you plan a later expansion with regard to children, the house needs a fixed staircase. Collect the many factors and accordingly see which building structure is then used. Bungalow would be out for me. And regarding the “problem” of bathroom usage, I think of everything else but not extending the already convoluted hallway further now. Long hallways are usually caused by poor planning. With an angled house, there are several possibilities to let daylight into a hallway. I would look for an architect if I were you.

Something like that belongs in the rough initial planning. You have the plot of land, which is sketched in detail with the surroundings, then you can assign rooms to the orientation.

Absolutely and necessarily. A target tree is part of the basic compilation as an absolute must— the accuracy of a Google Earth aerial image may suffice for this, one should never ever plan with decimeter precision anyway.

Long corridors are mostly due to poor planning.

And quite a few poor plans begin with an early and unyielding commitment to a specific house shape. Moreover, bungalows have a structural tendency to a higher "body fat percentage" in terms of corridors, which should be considered in the space budget.

If I were you, I would look for an architect.

An architect has already been found (and is now being disparaged again because she is not compatible with the desire for controlled residential ventilation). Turning away from her under this circumstance is a wise decision. However, one should not join a monastery just because the first kissed frog does not turn into a prince. Architects are not decorative trims for a house design but "the right tool," even if you can basically hammer with pliers. Would she even have been "the right one" for a timber house design?

An architect has already been found (and she is now being disparaged again because she is not compatible with the desire for a controlled residential ventilation system).

That's true. There was something like that here in the thread.
But as you say: then I won’t bother with a stuck plan and will make the hallway longer.
Besides, I was quietly irritated while reading the note about air conditioning and controlled residential ventilation by the words used. I looked for the sentence again and found it:

For example, the architect’s firm conviction that you don’t need air conditioning and controlled residential ventilation, although both are on our “Must-Have” list and she wanted to talk us out of it.

It may be that the architect has a firm conviction. Or too many clients whom she cannot satisfy. Or whatever. Or the client is resistant to advice, at least to discussion, or/and couldn’t really say why these things are a "must-have."
As a service provider, you face the client, the client presents their wishes, listens to arguments. Personally, I find the real reason for the split is missing ;)
Also: the basic structure is being adhered to here. Maybe that is more of a hindrance?