Thanks for the reply.
The proposed space between my neighbour and myself is empty - there is no need for access to that space. I basically want to align the new wall with the old wall as currently the new wall for the extension starts 20% into the back room .
Say this was to go ahead.. how much...
I have a semi detached house and my neighbour has a single story rear extension which comes right up to its boundary (next to our fence line). This isn't the issue. We also have a single storey extension but it is about a meter and a half away from my neighbour even though it was built before...