Who can witness a will?

The formalities come from the Wills Act 1837 and are unforgiving because they are checked after the only person who could explain them has died. A will in England and Wales is valid if it is in writing, signed by the testator (or by someone else at their direction), with the signature made or acknowledged in the presence of two witnesses present at the same time, each of whom then signs or acknowledges their signature in the testator's presence.

Who must not witness

• A beneficiary under the will, or the spouse or civil partner of a beneficiary. The will still stands, but the gift to that person fails entirely — an expensive, irreversible and surprisingly common accident.
• Anyone under 18.
• Anyone who lacks the capacity to understand what they are witnessing.

Executors may witness provided they take no benefit under the will. Neighbours, colleagues and friends are the usual safe choice.

The other capacity that matters

The testator must have testamentary capacity: an understanding of what making a will is and what it does, of the extent of the property being disposed of, and of the claims of the people who might expect to benefit. Where capacity may later be questioned — advanced age, a diagnosis, a will that departs sharply from an earlier one — have a medical practitioner assess capacity and make a contemporaneous note. That note is what defeats a challenge years later.

Two further rules catch people out. Marriage or civil partnership automatically revokes any will made beforehand, unless the will was expressly made in contemplation of that marriage — so someone who marries and dies before making a new will dies intestate. Divorce does not revoke a will; it treats gifts to the former spouse as having lapsed and removes them as executor, leaving the rest intact, which is rarely the outcome anyone actually wants.

Changes can be made by codicil, executed with exactly the same formalities, but for anything substantial a fresh will is cleaner than a stack of codicils. Store the original with your solicitor, a bank, or the National Will Safeguarding Service, tell your executor where it is, and consider registering it with the National Will Register so it can be found.