Dear Diyisfun, Alpcon and Masona,
I couldnt leave without a note of thanks to you for your replies.
Just to confirm:
Firstly we have enquired at our local planning office (my hubby actually works for the council, so we obviously had the best advice, thank goodness!).
We are advised that we do not require planning permission. However, we do require Building Regulations Approval, which we have yet to apply for and obtain before work can begin. We are also required by the Party Wall Act 1996, to advise our neighbour of our intentions in writing at least two months before work commences.
The plans have been drawn so that our neighbour will not gain any of our land, he will simply lose his skin of the party wall, therefore allowing an additional 4" of garden space on his side. Obviously if he insists, then we will reinstate his half of the wall, making the extension treble skin to exisiting wall height and double skin thereafter on our side.
So .... back to the original .... the plans show the guttering running over the 4" gap (created by removing his half of the party wall, effectively on his property) but our architect says there is a way of getting around this.
Seems 'Nige F' has hit the nail on the head.
Many, many thanks for your time and efforts and good luck with your projects.