I think I would be having words with a neighbour who decided to put up a 4.5m high structure 6 feet from my boundary and which stretched more than half-way down my garden. Aside from the impact on property value, if my garden is west-facing, that would put most of my garden in shade for most of the day. Permitted development be damned, that would not be acceptable.
Forty years ago I built almost exactly this - a fifty foot long workshop with a pitched roof reaching to about fifteen feet high at the apex, running along about two thirds of the boundary with my neighbour to the North, so it's on his South side.
The planning dept asked around and no-one had any objections, the next door neighbour's comment was "I can see more than I can have anyway".
Now, the current owners are perfectly happy with a high boundary wall since they like their privacy.
Takes all sorts...one man's meat and so on.
Martin.