ᐅ What is the height of my knee wall?

Sorry, from the point "2.3 m (7.5 feet)" and ".....the area (which area?) is smaller than three quarters of the area of the floor below" I completely lose track. I have read it 100 times and still don't understand.

Roughly sketch the cross-section as Doc-Dirk described to you. Then mark the points where the height starts to reach 2.30 meters (7.5 feet). The width between these points must not exceed three-quarters of your lower floor area. For example, if your house has a 10-meter (33 feet) wide living area, then the width in the attic floor must be less than 7.5 meters (25 feet).

Ultimately, this is just an approximate estimate, since you do not yet know the thickness of your ceiling and the rafters of the roof, and you have a maximum height of 9 meters (30 feet). The exact dimensions will only be determined through a drawing from the architect.

With a ridge height of 9 meters (30 feet) and a roof pitch of 30 degrees, assuming a "normal" house width, the ridge height of 9m (30 feet) should not pose any problems.