Attic size at 1.50m knee wall height

Wow, I am amazed that so many responses come in such a short time
The gable is on the short side.
Room height is 2.42m

Just briefly estimated again: With every meter of height, you lose one meter of width per side at a 45% roof pitch. 8.20m house width minus walls is roughly 7.50m interior width. Up to the room ceiling of the 2nd floor, roughly calculate one meter reduced usable width by 1m/side. That leaves 5.50m at ceiling height. Floor construction in the upper floor + body height of 1.80m together make maybe another 2 meters of height. That means the usable width decreases by 4m. Effectively, you therefore have no more than 1.50m width in which you can actually stand. Depending on floor construction, body height, and wall thickness, this could be less or somewhat more.

With a house width of 8.20 m (x 2 = 4.10), the ridge is 4.10 m higher than the eaves. With a 1.50 m knee wall (which is quite substantial in combination with a 45° pitch), that makes 5.60 m. Plus 2.85 m ground floor story height, 4.35 m eaves height and 8.45 m ridge height must be permitted. Starting from a 1.50 m knee wall, a room height of 2.42 m is reached 92 cm from the exterior wall, so the strip with full room height is 6.36 m wide. A room height of 2.00 m is already reached after 50 cm, so 7.20 m in width. With 94 %, this clearly counts as a full legal storey, as the slope does not reduce that.

The initial post sounds to me as if two stories are allowed as well. In that respect, it fits. It was more about how much attic remains.

You can also get advice from a professional and plan according to your individual wishes.

Individual wishes do not exist in show homes, and special requests are only available there for an additional charge.

A plan tailored exactly to your requirements is only available from independent planners (if they are up for it).
With that, you can still go to any house building companies.
...and if you ask different builders with the same plan, you get a feel for realistic prices. Meanwhile, you could simply entrust your planner.