Ground floor approx. 100 sqm, upper floor expandable (planned bathroom, 2 children's rooms, 1 storage room)

Some of the dimensions might not be quite right.

Probably the sloping roof. Can you estimate it based on the picture?

The house is being built now after all. Not much can be changed anymore anyway.
If the upper floor is really going to be expanded in a few years, the OP might have to work with dormer windows.

Everything could have been planned differently, but it was not desired.

Wind is not a reason to avoid breakfasting outside.
A hedge, a piece of privacy fence, and the terrace are protected from the wind.
And here you know what wind is and what cold is. We have 6 months of winter and 6 months of cold.

Probably the sloping roof. Can you estimate it based on the picture?

38? But I'm going by gut feeling and somewhat conservatively (2 meters ceiling height), because especially here, the attic space should be considered rather secondary, as it really feels quite oppressive. It would be different if the gable was fully glazed or the house twice as wide, but it is after all a bungalow roof on a house, and space up there is scarce accordingly.

I come from the coast, so it is only windy when you can lie down in the storm. We always had breakfast outside as soon as the sun shone and the temperatures allowed it. My parents also had direct access from the kitchen to the terrace. Now in our own castle, we always try to eat outside when the weather permits. Neighbors do the same.

The discussion about behavior in the wind with breakfast is very tedious here and does not contribute anything to the topic of the attic.

The roof pitch is 45° according to the sketch of the TE [45°]