Build a budget you can defend, not a hope

A real self build budget is a measured thing: the build itself, the professional fees, the services and connections, the VAT position, the contingency for what nobody can see yet - each its own line, each with a reason behind it. What sinks builds is the other kind: a total that started life as what you had, dressed up as what it will cost. When a quote comes in over a hoped-for number, nobody can say which line was wrong, so the difference gets absorbed on optimism. When it comes in over a measured number, you can see exactly where the gap is - and negotiate it, redesign it or walk away from it while walking away is still cheap.