How much contingency should a self build budget carry?

There is no single right number, and be wary of anyone who offers one without asking questions first. A contingency - the budget's stated allowance for what nobody can see yet - should be sized to where your build's genuine unknowns live: ground conditions before anything is dug, a design still moving, prices quoted months before they are paid, a specification with decisions still open. A build on a surveyed plot with a frozen design and a full specification can honestly carry less; a sloping site with half the decisions pending needs more, and pretending otherwise just moves the surprise into borrowed money. What makes a contingency work is less its size than its discipline: stated as its own line, sized for reasons you can say out loud, and never quietly spent on upgrades.