[Selanne_200]

I would begin by talk with your neighbour or the builder and see what their attitude towards the matter is like first. If they're planning to put up a new fence when they're done so it shouldn't matter too much.
In terms of legal liability, I would take pictures of the damage and hopefully you have some pictures of what your garden looks like and if worse comes to worse and they don't want to owe up to their damage, you can always take them to small claims. The key is you have to show that they encroached onto your side of the property line without your permission and based on a balance of probability that they had caused the said damage.