Large crack deeper than the plaster! How dangerous? Renovation costs?

Check if the crack is getting bigger. Simply screw 3 screws into the wall in a triangle and measure. However, this should be done over a longer period of time. If nothing moves, knock off everything loose and plaster over with a mesh.

or even simpler: plaster over with gypsum. If the crack moves, you will see it immediately.

I unfortunately cannot say how long the crack has been there. We have only lived here for 2 years. Best regards

And the crack was not there 2 years ago? Or was it less dangerous back then? The questions to be clarified are obvious. How long has the crack been there? Is the crack changing? Does the crack extend into the underlying components? Are there more of them? What static function does the wall have? This cannot be assessed with the pictures now.

TE writes they have only just discovered the crack now. However, they have been living there for 2 years. Given the appearance and size, I would almost guess that it cannot be missed during purchase / viewing? Suggests that it has only recently appeared. In that case, I would urgently have an expert take a look at it. Not only the plaster but also the stone is broken.

OP writes they have only just discovered the crack. But they have been living there for 2 years. Given the appearance and size, I would almost assume that you cannot overlook it when buying / inspecting? Suggests that it has only recently appeared. In that case, I would urgently have an expert take a look at it. Not only the plaster but also the stone is broken.

She writes that the crack is on the wall of the attic stairs, where one might not look very closely. That looks like a classic settlement crack to me. Stones and bricks crack like this under constant stress. A suddenly appearing crack due to an acute subsidence usually runs in the joints. The cracks also do not look too fresh. As others have already asked - what is below this floor, what is above? Can anything be seen from the outside? Has there been construction activity or groundwater lowering?

Thank you for the many answers. Our city is undermined by a salt mine and there used to be many explosions there. In addition, tanks often drive along the main road there. At that time, I did not observe the crack closely, but I have marked it now. However, I did notice it. I estimate the crack has been there for a long time. Definitely for the two years! I was there back then too. My neighbor says so as well. It is a terraced house. He says the crack has been there for decades. There is a staircase underneath. I was in the neighbor’s attic. That is the opposite wall. The crack is continuous. In the neighbor’s house, there is also a crack in the wall in the stairwell underneath. Not in ours.

I would take up 's suggestion and place a gypsum marker there. If nothing changes in the next few months, then have the plaster professionally repaired.