I think the legal height for a fence is 6 feet but if you're concerned about privacy I suppose there's nothing stopping you erecting a separate screen made of woven willow, or even close woven green netting material which is used as a garden windbreak, which you can't see through, nailed between two stout posts which start at the height your fence stops and goes up to ten feet.,. That way you you could will get some privacy while the trampoline is in situ, but can remove the screen when the trampoline has run its course, without damaging your fence..