What about a smelly compost heap on your side of the fence. They'll soon find somewhere else to sit.

Only kidding.

I'm guessing neighbours sitting outside is more likely to be a summer thing so (as others have said) try a small trellis with summer plants added to the top of the fence as you could argue isn't a permanent structure.

Are they on the shady side or the sunny side of the fence. If they are on the sunny side than you are more limited in your choice of plant but they're less likely to object over any issues regarding blocking their sun light.

Rather than ask (becomes difficult if they object) I'd be inclined to just stick up the trellis, leave for a short while and see if you get a reaction. If they say nothing then get some plants sorted as they will obviously be expecting it.

They may quite like the idea of a bit of colour anyway.

As for plants try some spring / summer / late (to cover the sitting out months) clematis or a fast growing climbing rose. Plant in large pots rather than the ground so you can move them if things don't go to plan.

Avoiding conifers, for an instant solution you could also try a young Eucalyptus tree in a pot. They grow like mad and can obviously be moved around.

I see no reason why not. Perhaps some sandbags, a couple of gun turrets and some barbed wire would do the trick.

No?

Ok, a 6" trellis then.....

The thing is most people people assume 6ft as that is the normal fence height. However 6ft 6 (2metres) is the legal height you can put up.
If the houses are private (and not ex council with their own stipulations) then you can go upto this height.
I think Hali is correct, height restrictions are applied to static things such as fences and walls. Plants and trees however do not come under this. I have a line of conifers in my garden which are 12ft and shaped into a long hedge so to speak. Never had any complaints. However they only shadow my garden, not next door.

Rather than trellis, how about putting up a chicken wire fench above the wooden one, you would still be able to grow your climbers along it, but in the winter when the sun is low it will not restrict any light.

Yes there is Hedge height

Using Leylandii etc as hedging is not encouraged, I'm going to plant a hawthorn hedge on my side of my back wall to my garden It will be kept to the height of the wall(6 feet)but I will encourage it to grow quite thickly as the "young people"who live in the house at the bottom of the garden have a habit of sitting on the wall & calling my dogs. A nice thick thorny Hawthorn will discourage them as well as being good for the wild life. A friend in the planning office & another in the enviroment office think it is a brilliant idea !!

be carefull selecting conifers, leylandii have been outlawed in some areas, they can undermine the foundations in buildings.

My other problem is that if I put trellis up I would have to put what ever climbing plant in a tub/container as I have concrete my side of the Fence - so which climbing plant would grow best in this situation?

Heard next doors Mother talking to her this morning saying that the decking isn't level & it's too high!!

Not that I'm a gardener, but a clamatis is nice in the right conditions and grows happily out of a pot. A creeping vaginia is also another good pot creeper. Visit your local garden centre and have a chat, they will be able to help you.

We planted Leylandii along our fence at the back of our house as we wanted privacy.You can plant them as long as you keep the height under control.The local coucil were out and complained about our front fence as they told me it was too high at 5ft,told me to cut a bit off.I said to her "Your having a laugh that a 4ft fence is going to keep a couple of Rotts in and are you going to take responsibility if they get out".She soon back tracked and left my fence well alone.
On the funny side of things you could always plant insect attracting plants hee! hee! Then sit back and watch them running around being chased by bees.Hee,Hee!Only kidding.
I agree trellis and clematis is the best idea and if memory serves me correctly clematis montana grows furiously.

Thank you for all your help - I feel a trip to the Garden Centre coming on..............