A codicil is a supplementary document that amends a previously executed Will. (Execution of a Will is when it is formally witnessed) The Will maker executes the codicil with the intention of adding to, altering, revoking explaining or confirming a Will, provision or part of a Will. As a subsidiary testamentary instrument, a codicil must be executed with the same formalities as a Will and becomes part of the will and must be proved with the Will as part of probate.
If your intentions have changed it may be prudent to have a new Will drafted. As a Codicil could make a small or large addition to or revocation of large parts of the Will, and in some instances can completely change the gifts under the Will.
A codicil is a document that makes reference to the original Will, describes the required change and is then signed and witnessed in exactly the same way as a full Last Will and Testament. A codicil confirming a will may have the effect of republishing the will or reviving an earlier revoked will. For example a codicil attached to the Will of a divorced testator could inadvertently confirm the provisions in favour of a former spouse.
In reality a codicil is an anachronism that arose at a time where legal stationers were employed to handwrite legal documents, including leases, wills and marriage settlements. Thanks to word processing software a new Will can be easily prepared and is preferable to a codicil.
A new Will is preferable for several reasons notwithstanding ascertaining that the Will referred to in the codicil is the latest unrevoked Will of the testator and still represents their wishes the person drafting the codicil must ensure the technical requirements as to the consistency of the language used in the codicil and the original Will crucially the date of the Will given in the codicil.
There may be reasons for a Codicil if the Will is complex, or the testator is very ill, a codicil may be appropriate if the alteration is straightforward. However, if your testamentary intentions have changed it is best to have a new Will made to avoid the complications that a codicil can bring.