What should a renovation contingency be, and what is it for?

Ten percent on a house built in the last twenty years, fifteen on anything older. It is not a buffer for changing your mind, it is for what the walls hide: wiring that does not meet current code, a drain with no fall, water damage under a shower pan, framing that was cut for a duct run decades ago. On a house of a certain age the contingency is spent more often than not. Deciding the number before the work starts turns a surprise into a decision.