A contract can be varied in one or two ways: either by a variation to the contract terms or the scope of works. The latter is the one that most of us will think of when the term variation is used.
Variation to the contract terms (or conditions) – also referred to as an amendment to the contract – amounts to the same thing. It is a change to the terms that the parties had agreed and accepted when the contract was signed.
To vary the terms and conditions of a contract the same degree of formality is required as was the case with the original contract. It must be in writing, signed by the respective authorised party representatives, and in same form as the original contract eg signed under hand or executed as a deed.
However, the more common understanding and indeed use of the term variation relates to a variation in the scope of works.
Attached is a Comprehensive Guide to Variations prepared by Baker McKenzie Lawyers.