party wall (vent)

[jkph173]

I have a fan vent from my bathroom fitted out through my side wall.The wall is built right up to the boundry and my neighbour is going to build an extension .His new wall will be right next to mine and has said that i have to move the fan vent as it will be blocked by his new wall.Can someone please tell me do i have to remove it ? can i just leave it there ? does he have to pay the costs if it has to be moved or is it up to me to pay. Thanks