As already mentioned above, I am very interested to know what amount could actually be saved through this change.
With a house width of 10m, we are talking about 5 sqm per floor, so 10 sqm less floor area in total.
On both floors, about 50 cm of wall on each side are saved - assuming a room height of 2.70 m, that is 2.7 sqm of wall area per floor, so a total of 5.4 sqm less wall area.
The items that make a house expensive remain the same. Nothing changes regarding windows, doors, heating system, installations, other technology, etc.
In my opinion, the material effort for 10 sqm of floor area, 5.4 sqm of wall area, and then a bit of roof area cannot make such a huge difference - especially since I have already read above that there are BUs that even build a wall a bit longer with no additional cost - they (usually) have nothing to give away either.
Therefore, I would strongly recommend that I check with the BU what savings this change would bring and then put this amount in relation to a changed floor plan, over which you might regret for a lifetime if in doubt.
Ultimately, of course, it is your decision.
Regards,
Dirk