The most difficult part of this, by far, is making the shed roof waterproof despite the ladder piercing it at eaves level. This matter should not be underestimated or the shed would quickly become unusable. A simple flap wont do because the ladder passes trough the hole in the roof. So the ladder would need a fully enclosing 'box' sealed to the shed roof with flashing, etc. The large triangle filling the open space between shed roof and obs. would almost certainly be an awful eyesore! A glass sided arrangement is not possible on the grounds of cost alone. However tempting, this is not remotely an easy option!
It might be possible to have the ladder rising from just inside the shed. This brings the ladder very close to the pier at the top. Weatherproofing of the shed wall is far more easily arranged with a simple porch between the two buildings. However, this idea needs a very large hole in the obs. floor for the user to climb through. With the joists cut away, serious thought would need to be given to the structural strength of the building. Reinforcement would be inevitable and difficult to arrange.
Another alternative is to have the ladder exposed in the foreground and approaching the building from the south. An access door would then be required in the SE face of the obs. octagon wall. This would leave the ladder exposed to the elements with all that entails. The SE veranda becomes the [potentially slippery?] landing for the ladder.
I have drawn in the ladder to scale to show the general arrangement. Any attempt to roof over the ladder for protection is almost bound to be visually, rather clumsy. Advantages include easy and direct access. Disadvantages include exposure to icy weather/snow. The ladder could be removable for security. Or a plank padlocked over its face to avoid uninvited guests on the first floor. This arrangement feels clumsy just looking at the image above. From an oblique angle it would look better thanks to the relative sophistication of the ladder.
Yet a fourth idea would be to partially hide the ladder in the gap between the buildings. This would require modification to both shed and Obs. due to the slightly greater width of the ladder. Arranging guttering might be a problem.
Unless I can think of something else, it seems the present, internal ladder arrangement is fixed. It is well protected from the winter weather and the building is secured by a solid, ground floor door.
Despite the sometimes thundery showers I was able to fix some floorboards with ss. decking screws. As well as blogging my endless thoughts on ladder layout, obs. access, and taking loads of photos, of course.
Now I have to seriously think about a rotating roof or dome. The pier is slightly higher than planned so a 3m semicircle is struggling to contain the dewshield. See image. I have marked head height at 1.8m with a flower pot on a pole on the right.