Mental capacity, dementia and will validity: what the law actually tests

A diagnosis is not a verdict on a will

Does dementia make a will invalid? No, not by itself. In England and Wales the law does not ask what condition appears in someone's medical records; it asks whether the person had testamentary capacity, the specific mental ability to make a will, at the time the will was made. A person in the early stages of Alzheimer's disease or vascular dementia can hold that capacity for years. Equally, someone with no diagnosis at all can lack it. The diagnosis raises the question; it never answers it.

This matters in two directions. Families sometimes assume a relative with dementia can no longer make a will, and give up when a valid will was still perfectly achievable. Others assume a will signed after a diagnosis must be worthless, when the law presumes exactly the opposite. This guide explains the legal test, how the Mental Capacity Act 2005 fits around it, the 'golden rule' safeguard solicitors use, and what happens once capacity really has gone. It is general information about how the law works, not legal or financial advice, and it covers England and Wales. Anything involving an actual disagreement over a will belongs with a contentious probate solicitor.

The legal test: four questions from Banks v Goodfellow

The test for testamentary capacity comes from the 1870 case of Banks v Goodfellow, and the courts have confirmed it remains the test for wills today. Stripped of the Victorian language, it asks four things of the person making the will:

Element	What it means in plain terms
1. Understand the nature of the act	They know they are making a will and what a will does: it directs who gets their property when they die.
2. Understand the extent of their property	They have a broad grasp of what they own: the house, the savings, the pension death benefits. Not a precise balance sheet, just the general shape and scale.
3. Appreciate the claims on them	They can call to mind the people who might reasonably expect to inherit (children, a partner, a dependant) and weigh those claims, even if they then choose to exclude someone.
4. No disorder of the mind distorting the decision	No delusion or disorder is poisoning their view of those people or driving gifts they would not otherwise have made.

Notice what the test does not require. It does not demand a perfect memory, an accurate valuation of every asset, or agreement from the family that the choices are fair. A will that meets these four elements is valid even if it is surprising, uneven or hurtful. The bar is understanding, not wisdom. GOV.UK's summary of what makes a will legally valid compresses all of this into the phrase "be of sound mind", sitting alongside the other formalities: being 18 or over, making the will voluntarily, in writing, signed in the presence of two adult witnesses. Our guide to the signing and witnessing rules covers those formalities in full.

Where the Mental Capacity Act 2005 fits in

The Mental Capacity Act 2005 sets out the general framework for mental capacity in England and Wales, and two of its principles shape everything about wills and dementia.

First, capacity is presumed. Every adult is assumed to have capacity to make their own decisions unless it is established otherwise. Nobody has to prove they are capable before signing a will.

Second, capacity is decision-specific and time-specific. The official guidance on assessing mental capacity is explicit that a person who cannot make a decision at one time "may still be able to make it at another time", and may be able to make some decisions but not others. In practical terms the guidance looks at whether the person can understand the relevant information, retain it long enough to decide, weigh it up, and communicate the decision in any way they can.

This is the legal foundation for what is sometimes called a lucid interval. Dementia is typically progressive but rarely uniform: many people are markedly clearer in the morning than the evening, or better in a calm familiar setting than a noisy one. A will signed during a period of genuine clarity, when the four Banks v Goodfellow elements are met, is valid, even if the same person could not have managed it the week before. The assessment attaches to the moment of the decision, not to the diagnosis or to an average of good and bad days. For wills specifically, the courts have held that the tailored Banks v Goodfellow test governs, with the 2005 Act principles running alongside rather than replacing it.

The golden rule: the safeguard most families have never heard of

Because capacity attaches to a moment in time, the strongest evidence of it is evidence created at that moment. That is the thinking behind the golden rule, guidance first articulated by the courts in the 1970s: when a will is made by an elderly person or someone who has been seriously ill, the solicitor should arrange for a medical practitioner to assess the person's capacity at the time the will is executed, record the assessment, and ideally witness or approve the will.

Two points about its status. It is a rule of good practice from case law, not a statutory requirement, so a will made without a medical assessment is not invalid for that reason. And it is not a guarantee: a court can still find capacity lacking despite a doctor's sign-off, or present despite the rule being ignored. What the golden rule does is fix a professional, contemporaneous record of the person's state of mind on the day, which is worth far more than anyone's recollection years later.

What following it looks like in practice

Take Derek, 81, recently diagnosed with early vascular dementia. His sons, Marcus and Paul, know he has never updated the will he made in the 1990s, which still names a long-dissolved charity and his late brother. Handled well, the process runs like this:

1. Derek instructs a solicitor promptly, while his condition is mild. Early action is the single biggest factor: capacity questions get harder, not easier, with time.
2. The solicitor meets Derek alone for the instructions, asks open questions about his family, his assets and his reasons, and keeps a detailed attendance note covering each element of the capacity test.
3. Given the diagnosis, the solicitor applies the golden rule: a GP or, in less clear-cut cases, an old-age psychiatrist assesses Derek against the testamentary capacity test on the day of signing and writes a short report.
4. The will is signed and witnessed in the usual way, and the capacity report and attendance notes are stored with it.

Derek's will is now supported by professional evidence created at the exact moment that matters. If anyone later wonders whether a man with dementia could validly have signed it, the file answers the question. The same logic applies to codicils: GOV.UK's guidance on making a will notes that updating a will means either a codicil or a new will, and each fresh document needs capacity at its own date.

Sensible safeguards when a relative wants to make a will

For families in this position now, the informational picture reduces to a short list of habits that protect everyone, including the person making the will:

• Act early. A mild diagnosis today is the easiest capacity assessment there will ever be.
• Use a solicitor rather than a DIY kit. The professional file notes are themselves evidence of capacity, and a solicitor knows when the golden rule is called for.
• Let the will-maker meet the solicitor alone. Instructions given privately show the decisions are the person's own, which protects the will and the family members who might otherwise be accused of steering it.
• Record the reasons for anything unexpected. If the will departs sharply from an earlier one, a note explaining why, in the person's own words, is valuable context.
• Store everything together. Will, capacity report and attendance notes, somewhere the executors can find them.

Our making a will checklist is a useful way to gather the decisions (executors, guardians, gifts, funeral wishes) before the first solicitor meeting, and the wills pillar guide walks through the whole process from instructions to storage.

When capacity has already gone: statutory wills

Once someone no longer meets the testamentary capacity test, nobody can make or change a will on their behalf privately. That includes an attorney acting under a lasting power of attorney: an LPA covers property and financial decisions during the person's lifetime, never their will. Our guide to lasting powers of attorney explains where an attorney's powers stop.

The only route left is a statutory will: an application to the Court of Protection asking the court to make (or change) a will in the person's best interests. It is typically used where someone with a serious brain injury, illness or advanced dementia has no will at all, or has one that has been overtaken by events. The application fee is £432, with a further £259 if the court decides to hold a hearing, plus any professional fees; the fee can be reduced or waived for applicants on low incomes or certain benefits. It is a slower and more expensive path than the person making their own will while they still could, which is the practical argument for acting early rather than a reason for alarm.

What happens if a will's validity is later doubted

Briefly, and purely as information about how the system works. A will that looks rational on its face and was properly signed and witnessed is presumed valid: the law starts from capacity, not from suspicion. If evidence emerges that raises a real doubt, for instance medical records showing serious impairment around the date of the will, the burden shifts to the people putting the will forward to establish that testamentary capacity existed. Courts decide these cases on the evidence about the specific person at the specific time, which is exactly why contemporaneous records and the golden rule carry such weight. An overview of the formal routes sits in our guide to how contesting a will works; anyone actually facing a dispute, on either side of it, should speak to a contentious probate solicitor rather than act on general reading.

For most readers, though, the message is the opposite of alarming: a dementia diagnosis is the signal to make or update a will carefully and soon, not proof that the chance has passed. If your family is at that point, an estate planning specialist can arrange the solicitor meeting, the capacity assessment and the paperwork as one joined-up job, so the will that gets signed is the one that stands.