Codicil - What is it and Why use one? A codicil is a document which modifies a will. It doesn’t replace the will but changes it by adding or removing gifts, changing executors etc. In most cases a new will would be better than a codicil. It would be better to have one consistent document than one modified document. Probably the only time to consider a codicil is when capacity is a potential issue. A will is not valid where the testator has lost capacity. They won’t be able to think clearly about the disposal of their property if they are mentally disabled. Where the will was done while a person had capacity but modified with a codicil at a later date if it is later ruled that capacity was lacking when the codicil was made then the codicil part would be invalid but the will itself may still be ok. This could result in a partial intestacy rather than a full one.