It almost feels like a cult that builders are better but the truth is that most builders are not the sharpest minds and simply follow instructions
It is a massive fallacy that just because a builder may not have had a university education that they can't spark together their brain cells.
It's also a fallacy that they simply follow instructions. They follow designs and specifications which are there to explain and detail the level, finish and end product, but then it is down to them to work out how to produce and deliver this. They are also responsible for the site and people management and a good builder will display elements of project management and quantity surveying to name but two additional areas.
I think the reason that many people think builders are simpletons is because anyone can have a crack at it... and if they manage it the once some think they can make money out of doing it on other peoples property too... until they get out of their depth and make a run for it.
Builders get a bad name, and builders are easy to come by, good builders are harder.
I would consider leeco as the exception rather than the rule when it comes to DIYing an extension. And suppose that he and his team have had some experience with joinery or this level of DIY, as well as getting appropriately qualified help for certain areas such as electrical wiring and central heating etc.
Having said that, a cost of approx 15k is not a ground breakingly significant saving over having a builder do the work I imagine that in Yorkshire, for a relatively small addition as in his pictures, you could probably have a builder complete the works for somewhere between 20-22k.... and have the work complete in three or maybe four weeks. Plus have all the work guaranteed. (We saw that leeco has been doing it since Nov 2012, and is yet to clarify if he has factored in their own labour costs - which whilst you may not pay should always be used to calculate build costs)
His extension in London would probably cost more (but then, so would a DIY build because material and delivery costs are higher too). But importantly, I know builders in the London area that can get reasonable sized loft conversions watertight in just two weeks!
THe other question that hasnt cropped up yet though is what will your home insurance (and mortgage lender) have to say about you DIYing a loft conversion. You my find that they will not cover you and will request someone with an appropriate accreditation or part of a builders scheme is used (ie; FMB or CIOB).