Dormer/ridge heightening for neighbours on/over property boundary

Questioner

We are about to buy a house (semi-detached), but now the neighbours have installed a ridge extension/dormer window on their house. However, this is on the outside of our inner wall. The overhang is still 20cm. Now we see some leaks inside and we suspect that it is coming from there. How can we best tackle this without immediately getting into trouble with the neighbours?

Lawyer

Don't you have an inspection report drawn up for (hidden) defects of the house before the purchase? This would be a good reason and then you also have proof.