Mental Capacity and Wills: What the Law Requires

The leading test for testamentary capacity comes from the 1870 case Banks v Goodfellow, and it remains the standard courts apply today even though it predates the Mental Capacity Act 2005 by well over a century. It asks whether the person understands the nature of making a will and its broad effects, understands the extent of the property they are disposing of, is aware of the people who might reasonably expect to benefit and can weigh their claims, and is not affected by any delusion that distorts their decisions about the will.

This is a functional test focused specifically on will-making, rather than a general assessment of mental health or intelligence. Someone can have poor short-term memory, need support with daily tasks, or have a diagnosed condition, and still have full testamentary capacity, provided they meet these four elements at the time they give instructions and sign.

The Mental Capacity Act 2005 sets out a general framework for assessing capacity for a wide range of decisions, from medical treatment to managing finances, and includes the presumption that a person has capacity unless it is established otherwise. While the two frameworks overlap in spirit, the specific Banks v Goodfellow test is what courts have continued to apply to wills, and it is more detailed on the particular things someone needs to understand and weigh.

In practice this means that someone assessed as lacking capacity for one purpose, such as managing a bank account, is not automatically treated as lacking capacity to make a will, and vice versa. Each capacity question is decision-specific, which is why a targeted assessment for will-making is more useful than relying on a general capacity finding made for a different purpose.

Dementia, a stroke, a brain injury or a mental health condition can all raise a question about capacity, but none of them automatically means a person cannot make a valid will. Capacity fluctuates for many conditions, and someone may have clear periods where they fully understand what they are doing, even alongside a diagnosis that affects them at other times.

This is exactly why the timing of the assessment matters so much. What counts is the person's capacity when they gave instructions for the will and when they signed it, not a general label attached to their health at some point during their life.

If there is any realistic doubt about someone's capacity, the safest approach is to arrange a formal capacity assessment at, or very close to, the time the will is made, usually by a GP or a specialist, following what is often called the 'golden rule' recommended by solicitors for elderly or ill testators. This creates contemporaneous evidence that can be relied on years later if the will is ever challenged.

Trying to establish capacity retrospectively, after the person has died, is far harder and relies on witness recollection, medical records made for other purposes, and expert opinion given without having actually examined the person at the relevant time. A short assessment at the time is a small step that can prevent a much larger dispute later.

Where there is any question over capacity, whether due to a diagnosis, age, illness or a family history of disputes, a solicitor should generally be instructed rather than using an online or DIY will service. A solicitor can take detailed instructions, form their own professional view on capacity, and arrange or record a medical assessment as part of the file.

This is not about assuming anyone lacks capacity; it is about protecting a will that may otherwise be vulnerable to challenge, and protecting the family from a dispute that a little extra care at the time could have avoided. If capacity is not in doubt at all, a straightforward online will remains a perfectly valid option.

A will can be challenged after death on the basis that the person lacked testamentary capacity when they made it, and if the challenge succeeds, the will is treated as invalid, meaning an earlier will (if one exists) or the intestacy rules apply instead. These claims can be lengthy, expensive and emotionally difficult for a family already dealing with a death.

The best protection against this is proper evidence gathered at the time the will was made, not arguments constructed afterwards. See our guide on can someone challenge my will for the wider grounds a will can be contested on, and avoid will being contested for practical steps to reduce that risk.