In tandem with planning a step up somewhere in the extension, I began looking at ways to increase the head-height of the doorway from the main house, and as such allow us to raise the floor level even further without compromising too much.
You can see the faint outline of an arch above the doorway, so today I set about investigating if that was structurally intact and whether it would help our problems.
The external arch is sound and strong, and has been supported by a large protruding stone on the right of the doorway. It was easy to remove the modern-ish door and frame, and peel this back to the original arch.
However, I suspect that when the 1880 extension was built, they did a fair amount of buggering about with this doorway and pulled out some of the internal side structure. They put in two oak lintels, about 3 inches deep and 6 inches wide, and bricked up the internal arch.
The lintels aren't sat on any kind of structure on the left of the picture below (except for a bit of lime holding them in place), so I had to think of the best solution to create a taller door and keep the structure.
I've decided to have an iron bar fabricated to support the internal arch - similar to the ones used to support curved fireplaces. I can cut this in to the stone on both sides and it will prevent the weak arch collapsing. Above the arch sits another oak lintel (shown in above photo) which is taking most of the weight of the 2nd storey walls.
More on this when the builder has confirmed my plan looks OK.