State the contingency and defend it

A contingency is the slice of the budget set aside for what nobody can see yet - ground that surprises you, prices that move, the small decisions that add up. Write it into the budget as its own stated line, sized to your build's real risks rather than picked because it sounds sensible, and then defend it: it is not a savings pot, and the moment it quietly becomes the kitchen upgrade fund, the next surprise comes out of money you have already promised elsewhere. A build that finishes inside its number almost always started with a contingency that was stated, sized and left alone.