Fencing and trellis

Hello everyone
My neighbour is going to have to replace the fence between our gardens. I shall ask him if he would put trellis along the top if I pay the extra. If he doesn't want to do this, can I put freestanding trellis with the posts right up against his fence? Or does it have to be a legal distance away?
I'll only do it anyway if he's happy, but just would like to know the position. Any thoughts and advice welcome
Thank you all!

 

I don't know, but i think it depends how tall it is, and how much it affects your neighbors sunlight