The construction company is building a basement smaller than in the factory plan

Was the survey engineer part of the contract? Give us some input.

Content-wise, you won't get much money out of that (except marginal amounts). The question of damage is therefore less important than how to proceed.

In my opinion, there is little that can be done here. Your site manager must have been asleep?

6.66m can be easily measured to the exact millimeter with a tape measure or folding ruler. No surveyor is needed for that.

Mistakes happen. If the dimensions are very important to me, then I measure myself.

Now to the solution of the problem. The staircase is presumably on the wall to the neighbor, so there should be no problem there. The masonry work above the basement can still be carried out as planned. The bricks simply protrude a few millimeters further. If you really want to, you could make the insulation of the basement slightly thicker. I call that a win-win situation.
If the brick's statics do not allow it, then an L-profile will be attached to the concrete.

Show me the section of the work planning. Transition basement / ground floor

These are concrete stairs that must be ordered in advance according to measurements, otherwise you lose unnecessary time.

Who ordered the stairs? You or the shell builder?

Who ordered the stairs? You or the shell builder?

I suspect it was him, and since they were already ordered according to plans, he now has a problem. But that's just a guess, because he still does not disclose all information (contractual relationship, ...).

Is there an architect, site manager, or similar, or will everything be commissioned independently?