The answer
Change Order (AIA Doc G701): A formal, legally binding written amendment modifying the original scope, cost adjustment (addition or credit), and revised completion schedule. Must be signed before extra work begins. Contingency budget guidelines: Newer homes (post-1990) / standard footprint require 10%–15% contingency. Older homes (pre-1978) or structural renovations require 20%–25% contingency to absorb concealed hazards (subfloor rot, outdated wiring, unvented plumbing, past unpermitted work).
Sources
This answer is drawn from the standards and guidance below. It is general information about how these systems work — not a diagnosis of your property.