Deviation of actual values from planned values

I don't know if I am in the right category, but I will just post my question here:

How much % deviation from the planned values must be taken into account or accepted?

Example: According to the plan, a room should be 350 cm wide, but it is only 335 cm. Would that still be within the tolerance range where nothing can be done about it?

Perhaps you can elaborate a bit on the background of your question. How far along is the construction?

Ok, and why is there this deviation? Who made the mistake and how?

No, you have landed in the moderated forum. Here, house construction companies are put under scrutiny.
Tolerance is, I believe, 2.5-3%.
However, of course, it is up to you to simply remeasure the shell construction here and there, wherever it matters.
The craftsman still has the opportunity to make corrections.

Planned values are raw construction dimensions, plaster is extra!
But that doesn't explain a 15cm deviation, admittedly.

Example: A room is supposed to be 350 cm wide according to the plan, but is only 335 cm. Would that still be within the tolerance range where nothing can be done?

That looks more like tolerance plus an octameter. Apparently, something has already been done, namely some mistake during the masonry work.