You don’t say what was there before, so it is difficult to say. For example if a concrete area was being ripped up and replace with garden beds then I think there is an argument it is a change of common property. If it is an existing garden area that is being fixed up, that’s a harder call.
As Kaindub says, if the work is well underway what are you hoping to achieve? Did you want them to reverse the work that has been done, incurring more expense? This is bearing in mind they can’t put back the tree?