A codicil is a document which amends a Will. Although there is no restriction as to when a codicil can be used, a codicil is generally suitable where minor amendments are needed to be made to a Will. For instance, a codicil can be used to replace a few paragraphs, change dollar amounts or update names of individuals named in a Will. If more substantial changes are required to a Will, especially structural changes, the use of a codicil can lead to confusion and errors, as the executor will need to refer to both documents in order to interpret the Will.

Another consideration, especially if a beneficiary is removed from a Will or their share is reduced, is that if a new Will is used, the previous Will will generally be unavailable to the beneficiary and they may not know that the change has been made. On the other hand, if a codicil is used, the beneficiary may have access to the Will and codicil, and will therefore see the change that was made.

A further consideration is the capacity of a testator. A person can only make a Will or a codicil if they have the capacity to do so. If a person's capacity is not certain, a codicil made while the person lacked the required capacity would be invalid, but the original Will would still be valid. On the other hand, if a new Will is made, the new Will would be invalid. This could lead to a serious issue as old Wills are typically destroyed when a new one is made. If this occurs and it is determined that a new Will was made while the testator lacked the required capacity, the testator will be treated as having died intestate.

Like a Will (and subject to some exceptions), in order for a Codicil to be valid, its signing must be need to be witnessed by two witnesses who are not beneficiaries or spouses of beneficiaries An affidavit of execution for the codicil must also be submitted to the court with an application for a certificate of appointment and which is often prepared at the time the Codicil is signed.