side infill

Hi, I'm hoping to have a single storey side return extension built on our terraced house. We hope to build along the party wall line but I'm struggling to figure out how the guttering will work without hanging over our neighbours land. I assume a box gutter on top of the wall may be a solution...

All, I'm new on here but going through a bit of a planning nightmare at the moment and would really appreciate any guidance or help the community can offer! I'll try to keep this brief but want to give the relevant details so please bear with me: I recently bought a 3 bed mid-terraced...