Well, it seems this thread is a little old, but I'm hoping that some of you are still out there and getting notifications.
I have found something amiss in the Approved Document J (thank you to "theoldun" for the links - really helpful). It concerns a similar situation to this one, but Approved Document J is not clear. I've called Gas Safe about it and they just refer back to Approved Doc J, saying it is a Building Regs issue.
I've spoken to our local Building Regs and they agree that the App Doc J is unclear, at least for my situation. Yet my situation must be a pretty common one.
So, I have a domestic combi boiler, fitted donkey's years ago. The flue went out horizontally and vented over a neighbour's garden (hence the similarity to this earlier thread). It is nowhere near anything in their garden, very high up and they were fully in agreement to it being done when we first put it in. It's not near any of their buildings or a patio, like with the original poster.
We have come to replace it and the plumber says that he cannot put a flue to vent over someone else's garden. The permission issue is put to bed - the neighbours have been asked again, agree and will give that in writing. All measurements in App Doc J relating to whether it is safe or not are met. It's not near any windows, door, gutters etc. So it is not that it would be dangerous. Of course, there's a "but" coming.
When the plumber called Gas Safe to check, they seemed adamant that Approved Document J says that no flue can be put "within 600mm of a boundary". But that's not the wording in App Doc J. It says "From a surface or a boundary facing the terminal". That would absolutely make sense if the flue were venting towards a boundary (surface wall or imaginary line), but in this case it is not. Moreover, it cannot be said that the boundary is "facing" the flue terminal, since in fact the boundary wall is behind the flue outlet (the flue comes through it, though it is our wall).
So we've got a situation where everything could be safe, the neighbours happy, but the plumber is (rightly) concerned that he will be doing something wrong. Gas Safe say they can just tell us the current regs, but there is a real chance to misinterpret them here. Moreover they say it is a Building Regs issue. Building Regs say they agree that the regs are not clear for this situation.
What to do? What to do? Nobody seems interested in trying to find whomever writes these documents, so that a definitive written answer/decision can be made.
And to top it all, I have seen many installations where a flue goes out of a party wall and vents (quite safely) over a neighbour's roof, for example.
Hoping for some constructive discussion on it from all you experts out there.