A fixed fee is an agreed price for a defined scope of work. It gives a basis for understanding the charge for that scope, while the engagement terms determine how changes and additional work are handled.
Check the included work, exclusions, assumptions, disbursements and the process for agreeing a variation. A fixed-fee description does not, by itself, mean every possible development in a transaction is included.
If the transaction changes, discuss the effect on the proposed work and charging arrangement before the additional work starts. Record the revised agreement.
See agreeing scope and fees for a practical engagement checklist.