This has been done to death on here and at the planning inspectorate. The land immediately adjacent to the development. Nothing to do with boundaries, "the land".
I think people get very set on keeping under 2.5m, but it's quite restrictive once you have insulation above and below the room. PP is relatively cheap - £250-300, you only need a scale drawing (which can be hand drawn) plus the site and location plans which you can download for £10 or so each. PP gives scope for a pitched roof, which can be a far nicer solution.
If you can build a 2.5m wall under PD tight to a boundary, then say a 2.3m eaves at the boundary and say a 3.3m ridge a bit away is hardly any more imposing or more shadowing.
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