I think you'll find 6' highest you can go, without upsetting neighbours. I think they could officially make complaint to council etc... as eye sore and above hieght specifications. I can't see make much difference if neighbours ok. I would ask them first tho. Make up some waffle you want to add a trellis 6" - 12" above fence as you would like to grow trailer plant (i.e. passiflora) or just tell them the truth - you want more privacy. Depends on your reason for 6ft plus??? :-�