Moving the chimney inside the wall - Advantages - Disadvantages?

Yes Katja, that may well be. For the move-in, due to lack of money, there probably won't be a fireplace installed at first, but later one of the type Camina S12 is supposed to be placed there. We haven't finally chosen one yet.

Why whole octameter steps? Is this dimension commonly used in construction? The drawing with the cutout in the initial post is with the shift of 59.5 cm as now proposed by the architectural firm.

Currently, the chimney does not protrude into the other children's room. But thanks for the hint regarding the change of the roof overhang. That would mean, for example, having 2 cm more roof overhang at the front and back than on the sides. This would change the angle and the rafter at the top would move out a little. Do I understand that correctly?

Due to lack of money, initially no fireplace will probably be installed at the move-in, but later one is supposed to be there in the style of Camina S12. We haven't made a final choice yet.

So could a pipe go there instead of a prefabricated chimney?

Why whole octameter steps? Is this measurement common in construction?

Basically yes.

The drawing with the cutout in the original post shows the shift of 59.5 cm as now proposed by the architectural firm.

Is that the one that also fits the roofer? - then everything is fine. Without the wall projection, only the "material change" and the expansion joint would remain in the children's room.

That would mean that, for example, there would be 2 cm more roof overhang at the front and back than at the sides.

More likely about ten centimeters so that it has the desired effect.

That would change the angle and the rafter at the top would shift out a bit. Am I understanding that correctly?

Almost yes - angles don't necessarily change, possibly only the ridge length and thus the points towards which the hips converge.

So could a pipe go there instead of a prefabricated chimney?

What kind of pipe? What would it be good for? (The prefabricated chimney is also supplied and installed by the general contractor)

Is that then the one that also fits the roofer?

Exactly. Before the roofer vetoed it today, the status yesterday was that the chimney would be pushed into the corner (just against the wall). Now the roofer wants the chimney—as can be seen in the original post—to be pushed a bit further and thus into the wall. This theoretically leaves 2.5 cm of sand-lime brick between the children's room and the chimney. But I estimate my general contractor will simply push the 2.5 cm further instead of milling out the sand-lime brick wall.

Flush in the same alignment would be most practical, and then either with a joint (if the surface is to be plaster) or instead drywall on this wall instead of the plaster (if it is to be wallpapered).

Keep in mind: Wall and chimney are two different elements that are only connected by silicone (as already mentioned above) and plaster. Otherwise, none. I would also build along the chimney!

I would still build along the chimney!

That means you would prefer moving the wall to changing the material?