Bumping this for the daytime crowd. I can answer some of the questions as my practice did the structural engineering.
Why is the house so bloody huge?
One of the reasons, bizarrely, is planning. He needed special planning permission as it is a new dwelling house in the countryside outside the development area. He needed to achieve a very low environmental impact, sustainable homes level 6 zero carbon (I think that classification has changed now), and be a 'mansion of architectural merit'. Thus it is very big. I do wonder whether even with a 20% reduction in its size, he would have achieved all that and be finished by now.
One thing that surprised me with the edit of the programme was no mention of the fire at their current house. They had a thatch fire and completely lost the roof, with considerable internal water damage from the fire fighting. It was insured and was all repaired (with a slate roof this time). That all delayed the new project significantly, and was probably why he started so late the first summer and set the chain of delays that followed. I'd have thought that would have made a good angle for the TV, but I suppose they had enough of an unusual story anyway.
Kevin McCabe is a mountain biker... but I've not seen him out on the bike for while as he seems to have spent every daylight hour on his build - that really came across in the programme.
He will definitely finish it - and it will make a great second programme in a year or so.