We moved into our house last year, neighbours have lived here a long time. Our gardens are on a slope so they're tiered, and all boundaries are shared (confirmed in deeds) so fences/walls are the responsibility of both neighbours and shared.
On one side of the garden the wall is a low wall and the height of it varies - you might think it varies in line with the tiers but it doesn't quite (at least from our garden) it's higher or lower at different points. Overall though there's no privacy, it's generally waist height or a little higher. Im fairly certain it was put in by a previous owner of our house.
When we moved in we spoke to the neighbours about putting either a fence up, or building up the wall (eg with a privacy trellis) and they said it was fine, though they were instantly concerned about whether they'd lose anything (eg if they got the 'nice side' or if we'd 'take' part of the wall as it's quite deep). They're polite but I can tell they're used to being a bit grabby - the previous owner didn't use the garden much and they created an access point to cut through the back (no legal right of way) which had to be blocked when it was sold, and they built an extension with overlooking windows that wouldn't be legal (the windows part not the extension itself) and it actually juts out over the boundary. They tried to dump lots of junk on us when we moved in to, eg furniture which was ready for the tip on the guise of being helpful - I genuinely don't think they were being helpful I think they just thought it was a way to get rid of stuff - we look quite young and they seemed to think we were moving in with nothing when we were moving from our first house, so with all our furniture, they seemed quite put out that we didn't want their old sofa, which was too old to be donated as it was pre fire retardant labelling!
The reason I'm giving the context isn't to slate them, they are nice neighbours, just that i know we can't rely on them to volunteer compromise - they are rather forward.
So the AIBU/WWYD - we've got quotes to build up the wall in places and add fencing on top. The question is how high. Our garden and the neighbours garden don't follow the same levels, so there are points that 6ft in our garden would be higher in there's. Would that be reasonable or unfair?
I have no way of knowing what the 'original' garden heights were. I suspect that they changed their levels because of the extension, it looks like they removed land at the bottom of their garden to build up the first half for the extension - I can't be sure though.
If we had the fence lower on those sections further down the garden (where theirs is lower) it would defeat the point for us as they can still see in a lot from the higher section.
I don't expect 100% privacy in the garden but at the moment it may as well be a communal garden its that open, so I do want to change it but preferably without starting a neighbour feud!