Cavity flashing costs: Who pays?

Questioner

I have a question regarding cavity lead; I own a house with an extension. This was installed during the construction of the house. My neighbors don't have an extension. Therefore, cavity flashing was installed during construction (in our exterior wall, on their roof). They now have a leak. Photos show that the cavity wall is damaged. Who should pay the €1,200 cost of replacing the cavity wall lead?

Questioner

The cause of the damage is the lead flashing, and that is your responsibility because the wall requires lead flashing due to the addition of a new structure. Therefore, you are responsible for the repair costs.