John Williamson wrote:

If it comes up almost as cheap as buying somwehere older and doing it
up, I'd maybe prefer it.

It will cost more than you think and far more than any armchair pundit
imagines.

This is good as a rule of thumb guide:

http://www.potton.co.uk/self-build/case-studies

By the time I've finished, a five bedroom house build with four
bathrooms and five reception rooms will end up costing me about 250,000
pounds, excluding the price paid for land.