Single-family house - convert the attic or leave it open (high rooms on the upper floor)

Hello everyone,

we are considering building a detached house 13 x 9 meters with dormers front and back for a family of six (4 children). Room-wise, everything basically fits on the upper floor with 5 bedrooms, 1 bathroom (no walk-in closet) - we are not exactly sure what to do with the attic now. Either leave the attic open and simply let the rooms end in the attic - the result would then be rooms with fairly high ceilings up into the attic which would offer further possibilities (building sleeping platforms with a carpenter, climbing wall, etc.). Or close off the attic, add a second stairway up, and have one or two more rooms.

Cost-wise it would probably be cheaper open, but there are pros and cons.

Disadvantages of an open attic:
Lighting would have to be all on the sloped ceilings, since there is no longer a ceiling.
Heating possibly more difficult because the rooms are quite tall.
No possibility to expand upward if another room is needed.

Advantages of an open attic:
Cheaper
Rooms can be used even more vertically, so more efficient use of living space.
Great sense of space

Does anyone here have experience?

Cost-wise it would probably be cheaper but there are advantages and disadvantages.

Who says it is cheaper?
Insulating a cold roof on the upper side of the ceiling of the upper floor is cheaper than insulating an entire roof structure.
If money does not matter, then you simply have to see whether the rooms are suitable for it at all. A steep roof pitch can take away any coziness from a narrow room.

Cannot be answered without a floor plan as to what is more sensible. Dormers at the front and back? (rather right and left). So, does the upper floor have knee walls, what height, what roof pitch?