Earlier quoted context omitted.
A problem is that you can't get a mortgage to build it yourself. Being cheaper in the end doesn't help you if you don't have the money right now.
You cant get a mortgage for self build in the USA? - its a definite market in the UK and you can save quite a bit.
I'm guessing the loan underwriters will be uncomfortable with the risk profile of a decade long project of self improvement, but what do I know?