A will that has been signed cannot be altered, either by crossing out or writing in new clauses, unless the new clauses are executed in the same manner as a will [see Wills Act 1936 (SA) s 24]. It is best to make a new will, but it can also be done by adding a codicil to the will. A codicil is an addition, in a separate document, to an earlier will. A codicil may confirm some of the contents of an earlier will and alter or add additional clauses to the will.

A codicil has to meet the formal requirements of a will (see Valid wills) and should refer to the original will and the date it was made. It need not be witnessed by the same people who witnessed the original will, but it must have a properly signed attestation clause.