What is a codicil?
A codicil is a document that amends an existing will without replacing it. It must be signed and witnessed with the same formality as the will and is read alongside it. Codicils suit small changes, such as replacing an executor or adding a modest gift.
Codicils are a survival from the era of typewritten wills, when retyping a document was expensive. Today, redrafting the whole will is usually cheaper, cleaner and less likely to be misread.
Each codicil must be stored with the original will, because a will read without its codicil produces the wrong result — and a codicil found without its will is meaningless. Multiple codicils compound the problem and invite construction disputes.
As a rule: use a codicil for a single, clearly separable change; make a new will for anything structural.