Under strict instructions not to make a massive mess in the house, the plan was to get Zoran, my joiner, to make up the main pieces in the garage, assemble the main structure in situ, then measure up for the more complicated pieces that go underneath.
Except, when we came to fit the 'frame', we discovered this:

There are hardwood treads to go on top and they're 30mm thick. He's forgotten to take that into account, and so every riser was 30mm too short, making the top step 210mm too low.
So, panic stations. 36 hours to fix before the girls get home...