On jobs with a degree of uncertainty (refurbishing old buildings, opening chases or floors without knowing exactly what is underneath), not including a contingency item in the quote is a risky bet that usually ends in conflict.
How much to set aside by type of project
As an indicative reference, on new-build or a well-defined renovation from clear plans, a contingency margin of 5-10% is usually enough. On refurbishments of old buildings or interventions where the real condition after demolition is unknown (old installations, hidden structure), it is reasonable to raise that margin to 10-20%.
How to communicate it without it sounding like "overcharging"
Explaining the contingency item as insurance against reasonably expected findings (not as a catch-all for anything) helps the client understand it as logical, not as an attempt to inflate the price without justification.
What to do if the contingency isn't used
If at the end of the work the contingency item has not been needed in full, communicating this transparently and adjusting the final invoice (or returning the difference, according to the prior agreement) builds a lot of trust for future work with that same client.
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