We used to have a 6m distance from the forest; today it is 10m.
Forest in the building law sense is what is marked accordingly on the forestry maps; whether there is a tree standing there or not. Which in turn also means that not everywhere where trees stand is automatically forest.
Therefore, have the corresponding maps shown to you, speak with the responsible forestry office. We cannot make legally valid statements from the ship.
If the property was sold to you as building land, and the seller knew that it falls within the forest distance zone and that you therefore cannot build there, then I would want to rescind the purchase due to deception (I hope I have written in enough detail now so that everyone understands... one should not assume that everyone who reads along also thinks along and can follow the thought process, even if it is not explained in detail down to every detail).