A contingency is not a prediction that something will go wrong. It is a buffer for conditions that could not be responsibly priced before work began.
Match the buffer to the unknowns
A cabinet repaint with no wall opening carries different risk from relocating plumbing in a century home. Review building age, evidence of leaks, previous unpermitted work, structural changes and whether destructive investigation was possible before the quote.
Separate contingency from upgrades
Use contingency for concealed damage, code corrections, verified quantity changes and other necessary work. Use a separate wish-list allowance for optional upgrades so the project does not consume its safety margin on decorative decisions.
Require documentation
A change should describe the condition, proposed solution, price, schedule effect and authorization. Photos and inspection notes help future owners understand why the work changed.
Return unused money to yourself
Unused contingency is not a signal that the project failed to maximize its budget. It is money that did its job by protecting the plan and can remain saved when the work is complete.