Have a read of this, taken from boundary problems website, hope this helps.
http://www.boundary-problems.co.uk/boundary-problems/fences.html
Fences as supports: Can I hang things on my neighbour's fence?
Fences as supports: Can I use my neighbour's fence as a support for my own plants?
Fences, appearance of: Can I paint my side of my neighbour's fence?
Only if your neighbour gives you permission to do so.
Leaning things against your neighbour's fence, hanging things on your neighbour's fence, even using your neighbour's fence as a makeshift retaining wall, will place a much heavier burden on the fence panels and supporting posts than they were designed to bear. The consequences of such actions are easy enough to predict and you will be liable for the cost of any repairs. On top of that you will still have to do, and pay for, the work that you did incorrectly and which resulted in the damage to your neighbour's fence.
In short, anything you do to your neighbour's fence without your neighbour's permission
- including staining, painting or applying preservative to your side of your neighbour's fence -
amounts to criminal damage.