Can old boundary stones be relied upon?

Isn't there the possibility to measure beforehand whether the distances roughly fit?

Theoretically yes. In practice it usually fails due to - lack of suitable measuring tools - local obstacles such as sheds or plants - incomprehensible surveying documents - misinterpretation of measurement results - lack of mathematical/geometrical knowledge

Would it be important whether they are correct?

Only if you want to rely on the markings. For construction, it is sufficient to temporarily indicate the correct position.

What happens if the stones are wrong?

If you do not check them and build based on the wrong stones, it depends on what is wrong, how big the error is, and how far the construction has progressed when the error is noticed. This ranges from toleration through compensation, overbuilding pension, re-parceling up to dismantling.

Do any claims arise from this?

A few centimeters on city land are usually negligible (unless the sidewalk no longer has the required minimum width). With neighbors, it depends on whether damage can be quantified. No court will bother with a loss of 0.2 m² of garden. But if they can no longer fit their prefabricated garage into the gap due to the overbuilding, one might have to cover the additional costs of individual construction.

I understand the theoretical implications; the question was rather directed at , asking what he expects from the boundary stones.

Thank you very much for your answers. My concern is that the house and the garage are built correctly and that no problems arise later, for example, having to pay the neighbor a lot of money because the garage was built on their land. We have now commissioned the survey office to measure the missing boundary stones and also to check the existing ones. Then we will be on the safe side.

Thank you very much for your answers.

Nice when an OP responds so thoroughly to the answers.

Good luck with the construction.