Log cabin with a clipped gable roof by the forest - improve planning?

He does not want to talk about money here, I assume that won't be the problem.

In general, floor plans of very large houses receive little approval and little constructive discussion here.

Nonsense.
If you don't even want to talk about money anonymously, it has to be clear that no one will believe you. No one is interested in discussing castles in the air.
Otherwise, large houses are very gladly and often discussed.

I didn’t want to waste anyone’s time with a pipe dream. The current plans are currently in the approval phase as a building application; we were under severe time pressure here, so we still need to submit changes - but at least we have now saved our expiring preliminary decision. My intention is therefore to once again take a critical look in a larger group at what can be improved or what we might have overlooked.
Is it important how much the house costs in euros for it to be taken seriously? If so: your tip of 650,000 euros was not bad, if you limit yourself to the house; my current calculation is within that price range.

Now to the construction conditions:

Light chimneys
The windows planned so far are 90x90 and barely integrable any larger, unless the roof is set back quite a bit at that point. Then larger windows and also another "light strip" between them could be realized. However, we find it visually rather unacceptable and have therefore looked at the light chimneys. Yes, the domes are not ideal on the roof either, but I guess solar modules are not exactly ideal either… we could live with it if it brings sufficient daylight into the room.

Upper floor hallway
Drawing in shelves is a good idea, I will try that. If in doubt, the option remains to partition off another room - as a playroom or something similar. Roof slopes should actually not be a problem at first, as they only begin at 2m height.

Basement granny flat
We discussed the idea yesterday. Two things still need clarification for us: on the one hand, we are not yet sure whether we get wastewater drainage in the basement without a lifting system; on the other hand, we only have daylight in two large rooms, theoretically for a living room/kitchen and a bedroom. Possibly we could expose one or two more windows on the west side, then this area could also be separated. But I think we have to decide on use – to install a wellness area and tear it out again in 10 years to build the granny flat, I would find that unpleasant.
So the only question left is about the entrance door; for a granny flat we would definitely have to provide another external door, otherwise they would always have to enter through the terrace doors. That would logically also be on the west side, right?

Wellness/Sauna
I think we can get everything we need into a small wooden house by the swimming pond. We have already built the first one; at 25 sqm it’s certainly not huge, but enough for the shower, sauna, a small relaxation area, and it can be well integrated with a terrace. On the rear part of the property, a holiday home is conceivable for a later time, which would also be suitable. That’s why I keep this part separate from the house. The main thing is that our bathroom upstairs becomes functional and cozy.

We discussed the idea yesterday. There are still 2 things we need to clarify: on the one hand, we are not yet sure whether we will get wastewater in the basement without a lifting station

If the washing machines are going into the basement anyway, you will need a lifting station, so just get one that can handle black water, then the bathroom and toilet will also work with it. The extra cost is manageable; we did the same. If you are building according to KfW55 or better anyway, with a granny flat you will get double the funding (2 housing units instead of one).

I think it’s hard to see, but the washing machine is located in the children's bathroom on the first floor so that the laundry can be washed where it usually accumulates. At first, we had everything in the basement with a laundry chute, but then we would have had to go up and down stairs again. Of course, it will be hung outside in the summer, we just can't completely avoid it; it’s a compromise.

The wastewater connection in the basement is problematic because after 2.5 meters there is rock, and we are not sure how easy it is to work with. A small wastewater treatment plant has to be built in the back part of the property, and the depth at which it is installed ultimately determines the outlet height at the house. A washing machine can pump the water easily up to one meter high, but a toilet’s drain is much lower. So if it’s not urgent and just a “nice to have,” it probably comes down to the extra cost. It’s also bad that it can’t be planned properly. I might have to have the treatment plant built first and then see how deep the wastewater is in the end. No one wants to make a statement without excavating.

KFW funding is very ambitious for pure wooden houses, or so I have been told. I’m still calculating, but I don’t really think it’s worth it. The effort is very high, the outcome uncertain.

I didn't want to waste anyone's time with a pipe dream. The current plans are currently in the approval phase as a building application,

Well, if that's the case....

I wouldn't (anymore) separate off the space at the top – at least to have the few windows for ventilation. But I would arrange the huge area the way I imagine a library with a cozy reading corner and a light shaft to prevent surprises.

What kind of shafts are those in the entrance hallway? Is the front door actually supposed to open outward?

I would leave out the tiny wall for the cabinet in the guest room – it just takes away light.

The depth of the kitchen cabinets looks so shallow. Of course, that might be deceptive because of the room sizes. Are those 60 cm? It certainly wouldn't hurt to allow 70 cm depth here. First, you could then realize extra depth if desired (which is nice for sinks), and second, you could also place an external appliance without it basically exceeding the frame.

The bathroom would need to be looked at again calmly. But it could be that there aren't many options because of the windows.

For the laundry, I would try to realize a laundry chute from the upper floor to the basement. Ideally, the laundry room opens onto the lawn where you can hang laundry in the wind. (…and don't you dare mention a dryer, anyway you need the exit)

An accessory apartment would be out of the question for me. Nobody moves in to my dream house! It's mine.
But a gym would be cool.

Find the hallway unsuitable as a library. Large room, very dark, the windows are just loopholes, little space for furniture. And if parents and/or children actually live in the house, the room is anything but quiet.

Take wider doors and possibly higher ones. They work better in large rooms.

I would have zoned the whole thing differently. Children’s area with 2 children’s rooms and the children’s bathroom there. Guest room, study, guest bathroom as one area. And a light-flooded library. Lots of shelf space, comfortable seating in front of the window, corner with the whisky collection.

I don’t like the staircase. It looks so cramped in.

Do you want everything inside to be wood-look as well?