but the construction of the house is such that the apex of this
extension is substantially lower than the main roof apex, which
also means that I am presented by a complete sloping face of the
roof on that corner, with no access into the extension roof.

Yes, some have a valley join, whilst some have lower floors and ceilings in
the "extension" to avoid the expensive roofing shenanigans required. Mine is
definately the former, whilst my mum's house has the latter.

Christian.