The short answer
A mental capacity assessment decides whether someone can make one particular decision at one particular point in time. Someone can lack capacity to manage a £200,000 investment portfolio and still have full capacity to decide what to wear or who visits them. A diagnosis such as dementia is never enough on its own to answer the question either way – each decision is judged separately, against the legal test below.
The legal test comes from the Mental Capacity Act 2005, which applies in England and Wales. If you need to know who can actually carry one out, skip to who can carry out an assessment. This guide also covers the situations that typically trigger one – including before a lasting power of attorney can be used, before an application for Court of Protection deputyship, and before certain medical treatment decisions.
Quick reference
| Question | Answer |
|---|---|
| What does it decide? | Whether someone can make one specific decision, at one specific time |
| Legal basis | Mental Capacity Act 2005, sections 1 to 3 |
| Starting assumption | The person has capacity, unless proven otherwise |
| The test | Functional test first, then diagnostic test (since A Local Authority v JB [2021] UKSC 52) |
| Who can assess? | No single approved assessor – a doctor for medical decisions, a solicitor for legal ones, family or care staff for everyday decisions |
| Can a diagnosis alone decide it? | No – capacity is decision-specific, not diagnosis-specific |
Sources: Mental Capacity Act 2005 – legislation.gov.uk; gov.uk – Checking mental capacity, verified September 2026.
The five principles behind every assessment
Before looking at the test itself, it helps to know the principles that sit underneath it. Section 1 of the Mental Capacity Act 2005 sets out five statutory principles that apply to every capacity decision:
- A person must be assumed to have capacity unless it is established that they lack it. Capacity is the default. No one has to prove they have it.
- A person is not to be treated as unable to make a decision unless all practicable steps to help them do so have been taken without success. This might mean explaining a decision in simpler language, using pictures or an interpreter, or choosing a time of day when someone is more alert.
- A person is not to be treated as unable to make a decision merely because they make an unwise decision. Making a choice other people disagree with is not evidence of incapacity.
- An act done or decision made for someone who lacks capacity must be done in their best interests.
- Before the act is done or the decision made, regard must be had to whether the purpose can be achieved in a way that is less restrictive of the person’s rights and freedom of action.
(Mental Capacity Act 2005, section 1 – legislation.gov.uk)
These principles matter in practice because they set the bar an assessor has to clear before concluding someone lacks capacity. A person cannot be found to lack capacity simply because they are elderly, have a diagnosis, communicate unusually, or have made a decision their family disagrees with.
The two-stage test
The Act sets out two questions that must both be answered before someone is found to lack capacity for a specific decision.
Stage 1: the functional test
Can the person understand, retain, use or weigh, and communicate the decision? Section 3(1) of the Mental Capacity Act 2005 says a person is unable to make a decision if they cannot:
- Understand the information relevant to the decision
- Retain that information for long enough to make the decision – briefly is enough; they do not need to remember it permanently
- Use or weigh that information as part of the process of making the decision
- Communicate their decision, by talking, sign language, or any other means, including a blink or a squeeze of the hand
Section 3(2) adds an important qualification: someone is not treated as unable to understand information if they can understand an explanation given in a way that is appropriate to their circumstances – for example, in simple language, using visual aids, or through an interpreter or advocate (Mental Capacity Act 2005, section 3 – legislation.gov.uk).
Stage 2: the diagnostic test
Only if the person fails the functional test does the assessor move to the second question: is that inability caused by an impairment of, or a disturbance in the functioning of, the mind or brain? Section 2(1) of the Act defines lack of capacity as being unable to make a decision because of such an impairment or disturbance (Mental Capacity Act 2005, section 2 – legislation.gov.uk).
This could be a temporary condition – such as the effect of medication, an infection, intoxication, or acute pain – or a longer-term one, such as dementia, a brain injury, or a learning disability. Both count. What matters is whether it is the cause of the person’s inability to make this decision, not the diagnosis itself.
Why the order of the two stages changed
Earlier guidance in the Mental Capacity Act 2005 Code of Practice suggested assessors should check for an impairment or disturbance first, then ask whether it affected the specific decision. That order was reversed by the Supreme Court in A Local Authority v JB [2021] UKSC 52, which held that the functional test (can they actually make this decision) should be considered first, with the diagnostic test used to explain an inability that has already been identified – rather than starting from a diagnosis and working outward from it. This sequencing keeps the assessment focused on the person’s actual decision-making ability rather than on a label.
Section 2(3): a diagnosis or appearance is never enough on its own
One provision is worth quoting directly, because it is the safeguard against the most common misunderstanding. Section 2(3) of the Act states that a lack of capacity cannot be established merely by reference to a person’s age or appearance, or a condition of theirs, or an aspect of their behaviour, which might lead others to make unjustified assumptions about their capacity (Mental Capacity Act 2005, section 2 – legislation.gov.uk).
In practice, this is why someone in the early stages of dementia may retain full capacity to make an LPA, write a will, or decide on their own care, while later needing an assessment for a specific, more complex decision.
Who can carry out an assessment
There is no single official “capacity assessor” role, and no central register of qualified assessors. Gov.uk guidance states that you can ask the person’s doctor or another medical professional to assess mental capacity, but in reality the person best placed to assess depends on the decision itself (gov.uk – Checking mental capacity):
| Type of decision | Who typically assesses |
|---|---|
| Making or using a lasting power of attorney | The certificate provider (see our LPA guide) at the time of signing; a GP if there is doubt |
| Medical treatment, including life-sustaining treatment | The treating doctor or clinical team |
| Court of Protection deputyship application | A doctor, psychiatrist, or other qualified practitioner, using form COP3 – see our deputyship guide |
| Day-to-day decisions – meals, what to wear, visitors | Family members or care staff involved with the person day to day |
| Complex or contested financial or welfare decisions | A social worker, or, in disputed cases, a psychiatrist or psychologist instructed for a formal report |
For everyday decisions, whoever is with the person – a carer, a family member, a support worker – is expected to apply the two-stage test themselves. For higher-stakes or contested decisions, a formal written assessment from a suitably qualified professional is usual practice, because it creates a record that can be relied on if the decision is later challenged.
When an assessment is triggered
A capacity assessment is not something that happens on a fixed schedule. It is triggered by a specific decision that needs to be made, where there is reason to doubt the person can make it. Common triggers include:
- Making or registering a lasting power of attorney. The person appointing an attorney must have capacity to understand what they are signing at the time – see our guide to lasting power of attorney. If there is any doubt, the certificate provider or a GP may be asked to confirm capacity before the LPA is signed.
- Before a health and welfare LPA can be used. An attorney under a health and welfare LPA can only act once the donor has been assessed as lacking capacity for the specific decision in question.
- Before an application to the Court of Protection. Anyone applying to become a deputy must submit form COP3, which includes a formal capacity assessment completed by a doctor, psychiatrist, or other qualified practitioner – see our guide to Court of Protection deputyship.
- Before a DNACPR decision is made on someone’s behalf. Where a person cannot decide for themselves whether they want CPR attempted, the clinical team must first establish they lack capacity for that decision before making a best interests decision – see our guide to DNACPR and ReSPECT forms.
- Before other significant medical treatment. Any time a patient cannot give or refuse consent to treatment, their treating clinician must assess whether they have capacity for that specific decision.
- When family or professionals disagree about someone’s ability to manage money or care. This is one of the most common triggers for a formal, independent assessment, often carried out by a social worker or an instructed psychiatrist.
What happens after an assessment
If the person has capacity
The decision is theirs to make, even if others consider it unwise. Under principle 3 of the Act, an unwise decision is not evidence of incapacity (Mental Capacity Act 2005, section 1 – legislation.gov.uk).
If the person lacks capacity for that decision
Whoever is acting on their behalf must make a best interests decision, following the principles in section 1 of the Act – including considering whether a less restrictive option would achieve the same outcome. In practice, this usually happens through one of the following routes:
- An attorney acting under a registered lasting power of attorney, if one exists
- An advance decision to refuse treatment, if the person made one covering this situation – see our guide to advance decisions
- A best interests decision by the treating clinician or care team, consulting family, friends, and anyone with legal authority to represent the person
- An application to the Court of Protection for a one-off order or ongoing deputyship, if no LPA exists and an important decision needs a legal decision-maker – see our guide to Court of Protection deputyship
- An independent mental capacity advocate (IMCA), appointed where the person has no family or friends appropriate to consult on a serious medical treatment or accommodation decision
Capacity can return or fluctuate
Because the test is time-specific, an assessment made today does not necessarily hold for the same decision next month. Conditions such as delirium, the after-effects of an infection, or the sedating effect of medication can temporarily reduce capacity, and it can return once the underlying cause resolves. Where possible, decisions should be delayed until the person can decide for themselves, rather than assessed at the point they are least able to engage (gov.uk – Checking mental capacity).
Common questions
Can a family member assess capacity themselves?
For everyday decisions, yes – anyone involved in supporting the person is expected to apply the two-stage test in a common-sense way. For anything with serious legal, financial, or medical consequences, a formal assessment from a doctor or other suitably qualified professional is the safer and more defensible route, particularly if the decision might later be challenged.
Is there a form for a mental capacity assessment?
There is no single universal form. The Court of Protection uses form COP3 for deputyship applications, completed partly by the applicant and partly by a doctor, psychiatrist, or other qualified practitioner. Hospitals and local authorities often have their own capacity assessment templates based on the same statutory test. What matters legally is that the two-stage test set out in the Mental Capacity Act 2005 has been properly applied and recorded, not which form was used.
What if I disagree with a capacity assessment?
You can ask for the reasoning behind the assessment to be explained, and ask for a second opinion from another professional. If the disagreement cannot be resolved and a significant decision is at stake, an application can be made to the Court of Protection, which can make a formal ruling on whether someone has capacity for a specific decision.
Does this apply in Scotland and Northern Ireland?
No. This guide covers the law in England and Wales. In Scotland, capacity is governed by the Adults with Incapacity (Scotland) Act 2000, which uses a different framework. Northern Ireland has its own Mental Capacity Act (Northern Ireland) 2016, which is being implemented in stages.
Related guides
Understanding capacity is the foundation for several other decisions covered on this site:
- Lasting power of attorney – appointing someone in advance, while you still have capacity, to make decisions if you later lose it
- Court of Protection deputyship – applying for legal authority to act for someone who has already lost capacity and made no LPA
- Advance decisions and living wills – recording, in advance, which treatments you would refuse if you later lack capacity to decide
- DNACPR and ReSPECT forms explained – how a capacity assessment feeds into decisions about CPR
- Planning ahead – the full set of things worth putting in place
Sources
- Mental Capacity Act 2005 – legislation.gov.uk
- Mental Capacity Act 2005, section 1 (the principles) – legislation.gov.uk
- Mental Capacity Act 2005, section 2 (people who lack capacity) – legislation.gov.uk
- Mental Capacity Act 2005, section 3 (inability to make decisions) – legislation.gov.uk
- gov.uk – Checking mental capacity
- gov.uk – Mental Capacity Act Code of Practice
- A Local Authority v JB [2021] UKSC 52 (Supreme Court ruling reversing the order of the two-stage test)