While the new house fits nicely into the gap we left for it Himself has sent it back to me with a request for some modifications.
On one side it is attached to the house next to it with the gable set at 90 degrees with the top of the roof tucked neatly under the gutter of the other property.
My design fits like a glove but I hadn't accounted for the 1mm thickness of the Wills slates sheet which would go on top - meaning it would no longer fit.
I tried to argue that we could simply lower the foundations to drop it down by the required amount but he wasn't buying it, so we agreed that I would attempt to cut down the gable ends.
Himself's eagle eye also noticed that he pitch of the roof lines of he upper and lower stories didn't match, so as well as taking a slice off the lower gables I have added a sloping sliver to the one above so they are now at the same angle.
So now everyone's happy.