I know this doesn’t help the original poster, but with a 30m distance, half of the houses in our neighborhood wouldn’t be allowed to stand. Presumably, this is because in the Black Forest it usually doesn’t work any other way or it is regulated in the development plan. Many plots of land are only separated from the forest by a small access road and a ditch.
Out here in the countryside, new development areas are also laid out next to forests. There certainly aren’t 30-meter buffer zones there. Nobody could afford a property like that; it would be unused garden land.
I would try to reverse the purchase due to deception.
Why? Apparently it’s “only” about overhanging branches... which were probably noticeable during an inspection. And the unpleasant consequence of grandfathering; one could have informed oneself earlier. Grandfathering is simply not a new phenomenon...
One must also be aware that if you want to build next to a forest (it’s there, you see it in its full glory and probably also appreciate this natural location of the property), you have to expect possible disadvantages as well. Of course, a tree might fall sometimes, more leaves/needles might fall onto your own property, or animals might cross your property. Of course, it would be nice if the forest owner showed understanding and parted with a row of trees. But it’s not like he wants to – and you probably can’t force him... because of this grandfathering.
The authority does have concerns and one should come to an agreement before the building permit – but it doesn’t read as if a permit won’t be issued at all if nothing happens.