Find the notice clause and its days. Put the due date in the change log the moment a change is logged.
How it flows
Keep the notice short: what changed, why it is extra and an estimate. A rough number now beats an exact one later.
Send it to the person named in the agreement, by the method it names. Keep proof of sending.
Follow up with a priced variation once the hours are known.
Give notice of a fee variation in writing, within the agreement's time, naming the change, the date and the likely cost. Do it the same week the change arrives.
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