Brighton and Hove prices - a typical example, acquire a bungalow with 2 bedrooms for £450k or less in a popular area, then spend £120k converting it into a 4 bedroom house. More attainable than having to spend £675k on the finished product.
Not a builder, but with experience of a similar project and with builder connections, I think that's doable.
Get several quotes.
If you're able to get a non-VAT registered builder, for example you engage and pay the various tradesmen under the oversight of the main builder, you'll save...