This was just me wondering, no one is particularly right or wrong.
Obviously, it depends on skills, time, money and attitude.
We are doing it block by block as our budget is <200k for 240sqm barn conversion.
This requires 2.4m underpinning and we couldn't even get anyone to quote on doing it. So, rollup your sleeves and get on.
Hence we are doing it in phases, like building 5 mini houses until we get to roof level.
I'm not sure what happens then though.
One real advantage though is you've got time to make decisions and rectify any mistakes when they happen (we've made a few)