A will is only valid if you had the mental capacity to make it. That sounds straightforward, but it's the area where a surprising number of wills come unstuck — not because someone wrote a bad will, but because they waited too long, or wrote one in the early stages of an illness that later progressed.
Capacity isn't a binary you either have or don't. It can fluctuate, decline gradually, or disappear suddenly after a stroke or accident. And once it's gone, options narrow sharply. This guide explains what testamentary capacity actually means, why timing matters more than most people realise, and what happens if capacity is lost before a will is written or updated.
What is testamentary capacity?
Testamentary capacity is the legal term for being mentally capable of making a will. The classic test comes from a 19th-century case, Banks v Goodfellow, which is still the leading authority. To have capacity, you must:
- Understand the nature of the act — that you are making a will, and what a will does.
- Understand the extent of your property — broadly, what you own and what you're disposing of.
- Understand the claims to which you ought to give effect — who might reasonably expect to inherit (spouse, children, dependants).
- Be free from any disorder of mind that would influence your decisions in ways you wouldn't otherwise have made them.
You don't need a perfect grasp of every detail. Someone in the early stages of dementia might still have capacity on a good day. Someone with severe depression might not have capacity even though they appear lucid. The question is functional, not diagnostic.
The Mental Capacity Act 2005 also applies, and provides a more modern framework, but for wills the courts continue to apply the Banks v Goodfellow test alongside it.
Why timing is everything
Most people put off writing a will. The average person doesn't write one until middle age, and many people never do. The trouble is that the conditions that most often take capacity away — dementia, stroke, advanced illness — usually arrive later in life, when many people are still telling themselves they'll "get round to it."
The cruel arithmetic is this: the years when you're most likely to need a will are also the years when you're most likely to lose the capacity to write one. Once capacity is lost, you can't write a new will, you can't update an old one, and you can't sign a power of attorney. The choices narrow to whatever the existing situation is — including, in many cases, the rules of intestacy.
If that thought prompts you to act, start your will online — most people finish in well under an hour, and you can revisit and update it as your situation changes.
How capacity is assessed in practice
Capacity to make a will is a clinical and legal judgment combined. In a contested situation, the court looks at:
- Contemporaneous evidence — notes from the person who took instructions, records of conversations, drafts and revisions.
- Medical evidence — GP notes, specialist letters, care home records, any cognitive assessments around the relevant time.
- Witness evidence — what the witnesses to the signing observed, what family members reported, what the will-writer said and did.
- The "Golden Rule." Where a will is being made by an elderly or seriously ill testator, best practice is to have a doctor present at the signing, who can record their view of the testator's capacity. This isn't a legal requirement — it's a precaution against later disputes.
For someone in the early or middle stages of dementia, a capacity assessment by a clinician at the time the will is signed can be the difference between a will that holds up and a will that gets challenged.
What can be done if capacity has already been lost?
If someone has lost testamentary capacity, they can no longer make or change their own will. But there's one route left: a statutory will, made by the Court of Protection on behalf of someone who lacks capacity.
The court can authorise a will to be made in the person's name where it is in their "best interests." Typical situations include:
- Someone who never made a will and whose existing intestacy distribution would be unjust (for example, an estranged spouse would inherit everything and a long-term carer or partner would receive nothing).
- Someone whose existing will is significantly out of date — the named beneficiaries are dead, the executors have moved abroad, or the distribution no longer fits the family situation.
- Someone whose estate has changed dramatically, for example through a personal injury award or inheritance.
Statutory wills aren't quick or cheap. They require an application, evidence about best interests, notice to interested parties, and sometimes a court hearing. The process commonly takes months and the cost runs into thousands of pounds. They are a last resort, not a substitute for writing a will when you're well.
The link between wills and powers of attorney
Capacity affects more than just your will. If you lose it, you also lose the ability to manage your own bank accounts, sell property, or make financial decisions — unless you've signed a Lasting Power of Attorney while you still had capacity.
A Lasting Power of Attorney is a separate document from a will, but the same logic applies: it has to be set up while you still have capacity. Once you've lost it, your family has to apply to the Court of Protection for a deputyship instead — slower, more expensive, and supervised in a way attorneys aren't.
We've focused this article on wills, but the practical message is the same for both: don't wait. The window to act is open until it isn't.
Common scenarios where timing went wrong
Without naming names, the kinds of situations that arise repeatedly include:
- A widower in his late 70s who'd "always meant to update the will" after his wife died is diagnosed with vascular dementia. By the time the family realises an updated will is needed, his capacity is contested.
- A woman with early-stage Alzheimer's writes a will but doesn't have a contemporaneous capacity assessment. Years later, after her death, the new will is challenged by relatives who would have inherited under the previous one.
- A man who never wrote a will at all has a stroke, loses capacity, and dies six years later. His estate passes under the rules of intestacy to relatives he hadn't seen in decades, while his long-term unmarried partner — see our cohabiting couples UK will guide — is left with nothing.
The thread running through all of these is that they were avoidable. The action that needed to happen was straightforward; what got in the way was timing.
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How to protect yourself and your family
The practical takeaways are simple, even if they require you to actually do something:
- Write a will now, not later. If you don't have one, the cost of getting it sorted is small. The cost of not having one is potentially enormous.
- Update your will after major life events. Marriage, divorce, new children, property purchases, the death of a beneficiary or executor.
- Sign Lasting Powers of Attorney at the same time. Both for property and financial affairs and for health and welfare. They're cheap insurance against losing capacity.
- Get a capacity assessment if there's any doubt. If you're writing or updating a will after a diagnosis of dementia, depression, or any condition that affects cognition, a brief capacity assessment from a clinician at the time of signing is well worth it.
- Don't lock the will away unfindable. Tell your executors where it is. Capacity at the time of signing is no use if the document can't be found after death.
For complex estates, we recommend you seek assistance from a Trusted Hands Advisor or your own legal advice — particularly if you're writing a will alongside a recent diagnosis or while taking medication that affects cognition.
If your situation is straightforward and you're well, our guided will builder is the simplest way to get a valid will in place quickly. You can update it later as life changes, as long as you still have capacity.
> Ready to start your will? Trusted Hands turns these decisions into a 15-30 minute guided builder. Start free → — only pay when you download.
Frequently asked questions
Can someone with early-stage dementia still write a valid will?
Often yes, but the will is more vulnerable to challenge. Best practice is to have a contemporaneous capacity assessment by a clinician at the time of signing, and detailed notes from the person taking instructions.
What is the "Golden Rule"?
It's a guideline (not a strict legal requirement) that wills made by elderly or seriously ill testators should be witnessed by, or with the involvement of, a medical practitioner who records their assessment of capacity. Following it makes the will much harder to challenge later.
What's a statutory will?
A will made by the Court of Protection on behalf of someone who has lost capacity, where it's in their best interests. It's the only route to making or updating a will once capacity has been lost.
Can I challenge a will on grounds of lack of capacity?
Yes. A will can be challenged for lack of testamentary capacity (or for undue influence, lack of knowledge and approval, or improper execution). The challenger generally has to put forward evidence; the burden then shifts depending on the circumstances. These cases are fact-specific and often expensive — see our guide to will mistakes UK for related issues.
Does a Lasting Power of Attorney let my attorney change my will?
No. Attorneys can manage your finances and (if authorised) your health and welfare decisions. They cannot make or alter your will. Only you (with capacity) or the Court of Protection (through a statutory will) can do that.
Ready to write your will?
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