Depends on the state of the current construction; how much of it can be used, how much needs to be rebuilt; the standard of internal finish required etc. I put a roof on a self build in MK a few years back but the guy ran out of money because his wife had spent over £75k on bespoke kitchen and bathroom fittings! They had no carpets for two years plus half the house wasn't finished off with internal joinery.
Assuming the structure is completely sound, you could probably get it done for under £100k if you do a lot yourself.
Whatever you think your budget is multiply it by 3 and you should be OK
Barn conversions are usually quite build specific, i.e. it has to stay within the design of the original structure, so knocking it down and building a Potton may not be allowed by the planners.
