You can't make the house smaller on the inside just because of the bricks. Since when has there been something like that?
The point of view is not mine, and I agree with you:
You would only have to do that if the building plot was used to the last millimeter. But that doesn't play a role here at all. It is not for nothing that the payable area is based on the "interior square meters". Everything else would be new to me.
But for the principle "the wall thickness affects the ratio of footprint to living area," it doesn't matter, because this "suffers" when the walls get thicker. Of course, I would also rather add the wall thickness "outside," if only so that the ceiling panels and their spans do not need to be recalculated. Town & Country has done it the same way as far as I know when they went from 24 to 30 cm standard wall thickness. Precisely because it makes more sense. But builders usually calculate the other way around: building window minus fortress walls equals the space available for living. From a technician's perspective, it is different: with an existing house design, it is better to keep the exterior wall - interior edge and let the house grow outward with the thicker walls; with a newly created house design, it is better that the outside edge of the exterior wall is congruent with the octameter grid.