I did treat the first 11 rows of timbers before construction, and washed large quantities of the wood preservative stuff down the corner joints and all around the door and window frames. You can see the slight discolouration to the bottom half of the build in my last photo above.
I will add another 2 or 3 treatments before painting it.
I agree about the water getting in under the walls. Luckily I’ve been able to see that over the last few weeks before putting a floor down. I had replaced the supplied bearers with bigger ones which means I have sufficient clearance to put paving slabs all around the shed sloping away from the walls. In doing so I will leave a small gap between the exposed bearer and paving slab edge.
Before I do that I will seal the gaps under the bearers. I’m currently thinking about using a mortar gun to run a 20mm bead of cement along and under the bearer frame (inside and out). I then intend to paint the outside bead with roof sealant.
Guttering to water butts should complete the water defences. I certainly won’t be putting the floor down until I know water is not getting in.
The Dunster House design allows for movement around the door and window frames - which have adjustable bolts like the storm braces have.
I have got the roof boards on now, and very pleased with that result. I have further edged and reinforced the roof all the way round. I will be putting insulation and osb boards on top. The shed currently has a sheet of dpm on the roof because of the ongoing rain. This will in due course be stapled to the osb boards before the final roof covering is attached. Undecided between shingles or roof panels at the moment.
I do like the vertical planks protecting the log ends, I will give that some thought.