Window handles with position feedback or window contacts?

Surface mounting is the retrofit option.

This is mainly intended for the basement. Since it is a waterproof basement, I have heard that I am not allowed to put boxes or similar items into the concrete walls (otherwise we cannot maintain the waterproof basement).

Yes, that's true, but you still need wiring. Surface-mounted?

Surface-mounted?

Yes, that seems to be the only solution. I am looking for ideas on how to make it as neat as possible.