A Codicil is a formal legal document that amends an existing Last Will and Testament without revoking the Will in its entirety. The word 'codicil' derives from the Latin codicillus, meaning a small writing tablet — it is, in essence, an addendum or supplement to the original Will. A Codicil allows a Testator to make relatively minor changes to their testamentary dispositions — such as replacing an Executor who has died or moved abroad, adding a new specific legacy, removing a gift that is no longer appropriate, or changing the shares in which the residuary estate is distributed among beneficiaries — without the expense and delay of executing an entirely new Will.

In England and Wales, a Codicil must comply with exactly the same formal requirements as the original Will under section 9 of the Wills Act 1837. This means the Codicil must be in writing, signed by the Testator in the simultaneous presence of two witnesses, and attested and subscribed by both witnesses in the Testator's presence. The witnesses must not be beneficiaries under the Codicil or the original Will (or the spouses or civil partners of such beneficiaries), or they will forfeit their entitlement under section 15 of the Wills Act 1837. There is no requirement that the witnesses know the nature of the document they are witnessing.

A validly executed Codicil has one significant additional effect: it re-publishes the original Will as of the date the Codicil is signed. The doctrine of republication, well established in English law, treats the Will and Codicil as if the Will had been remade on the Codicil's date. This means that property acquired by the Testator between the date of the original Will and the date of the Codicil can pass under the Will; and it also means that the Codicil can in some circumstances cure defects in the original Will that might otherwise have rendered it invalid.

The administration of estates in England and Wales following the Testator's death is governed by the Administration of Estates Act 1925, under which Executors obtain a Grant of Probate and administer the estate. When applying for probate, the Executors must produce all testamentary instruments — the original Will and any and all codicils — to the Probate Registry. Only by reading the Will and all codicils together can the Probate Registry and the Executors understand the full extent of the Testator's testamentary intentions and distribute the estate accordingly.

A Codicil is the appropriate document to use when the changes you wish to make to your existing Will are limited in scope and the original Will remains broadly suitable. The most common circumstances requiring a Codicil in England and Wales are as follows.

First, and most frequently, a change of Executor. An Executor may have died, relocated abroad, lost mental capacity, or declined to act since the original Will was made. A Codicil allows the Testator to revoke the appointment of that person and substitute a new Executor, without disturbing any other provision of the Will. Under section 36 of the Trustee Act 1925, a substitute trustee can be appointed by the person nominated in the original trust instrument, but where the original Will is silent on this point, or where the Testator wishes to be certain that a specific individual acts, a Codicil confirming the new appointment is the safest course.

Second, a change to specific legacies or gifts. The Testator may wish to add a new item (such as jewellery acquired after the Will was made), remove a gift (because the asset no longer exists or the intended recipient has predeceased), or change the beneficiary of an existing specific legacy. A Codicil is an efficient way to achieve these changes without re-executing the entire Will.

Third, a change to residuary beneficiaries. The Testator may have had a falling out with a beneficiary, wish to include a new family member born after the Will was made, or wish to increase charitable giving. Changes to the residuary estate can have significant Inheritance Tax implications — notably where charitable gifts are increased to reach the 10% threshold for the reduced 36% IHT rate under Schedule 1A of the Inheritance Tax Act 1984.

Fourth, a change of guardian for minor children. The nominated guardian may have died, become incapacitated, or been assessed by the Testator as no longer appropriate. A Codicil appointing a replacement guardian under section 5 of the Children Act 1989 is a straightforward amendment.

A Codicil is generally not the right instrument where: the original Will is old and the Testator's circumstances have changed significantly; the Testator has married or formed a civil partnership since the Will was made (which revokes the Will under s.18 of the Wills Act 1837); or the changes are so extensive that reading the Will and Codicil together would be confusing. In those cases, making a new Will is strongly recommended.

A legally valid and effective Codicil for England and Wales requires several key components, each of which mirrors the requirements for a valid Will under the Wills Act 1837.

The first is clear identification of the Testator and the Will being amended. The Codicil must state the Testator's full legal name, address, and the date of the original Will. Where more than one codicil has previously been made, the Codicil should confirm whether prior codicils remain in force or are being amended.

The second is testamentary capacity. The Testator must have the same testamentary capacity as is required for making a valid Will — the capacity defined in Banks v Goodfellow [1870] LR 5 QB 549, requiring the Testator to understand the nature and effect of a Will, the extent of the property being disposed of, and the claims of those who might expect to benefit. If a Testator who lacked capacity when the original Will was made subsequently executes a Codicil with full capacity, the doctrine of republication may validate the original Will: Re Goods of Jackson (1871) LR 2 P&D 214.

The third is precise drafting of amendments. Each amendment should be clearly numbered and should either expressly revoke the original Will provision being changed, or add new provisions that are unambiguous in their effect. Where a clause is being replaced, the new text should be set out in full. Courts construe Wills and Codicils objectively from their words and surrounding circumstances, and ambiguity can give rise to costly construction actions.

The fourth is the confirmation clause. The Codicil should expressly confirm that, save as amended, the original Will (and any prior codicils) is ratified, confirmed, and republished as of the date of the Codicil. This 'saving clause' is essential to make clear that the Codicil is supplementing, not revoking, the original Will.

The fifth is the attestation clause and dual witness execution. Under section 9 of the Wills Act 1837, the Codicil must be signed by the Testator in the simultaneous presence of two witnesses, who must then attest and sign in the Testator's presence. A properly worded attestation clause — confirming that both witnesses were present when the Testator signed — creates a rebuttable presumption of due execution under Sherrington v Sherrington [2005] EWCA Civ 326. The witnesses should record their full names, addresses, and occupations to assist identification if the Codicil is challenged.

Finally, safe storage is critical. The Codicil must be stored with the original Will so that both documents are found together at the Testator's death and produced to the Probate Registry with the application for a Grant of Probate. Executors should be informed of the Codicil's existence and location.