How do I keep changes from wrecking the budget?

Builds are rarely wrecked by one big change; they are wrecked by thirty small ones, each waved through in a doorway, none priced at the time, all arriving together in the final account. The defence is process rather than willpower. First, the rule with no exceptions: every change - a variation, in contract language - gets its price in writing before the work happens. Second, a register: one sheet where every change lands with its date, description, price and status, so the story is never scattered across text messages. Third, the running total: before saying yes to anything new, look at what all the yeses so far add up to, because each change is only ever judged affordable against the original budget, never against the budget as it now actually stands. That habit alone saves more money than any negotiation at the end.