Families arrive at the word capacity by different routes: a stroke, a dementia diagnosis, a serious head injury, a long-standing learning disability, or a brief episode of illness that clears. What they usually assume is that capacity is a single switch, and that once a doctor has flicked it, decisions pass to somebody else.

The law does not work that way. Under the Mental Capacity Act 2005, capacity is assessed for a particular decision at a particular time. Someone may lack the capacity to sell their house while retaining the capacity to decide what to eat, who visits, or which coat to wear. The legislation is deliberately built to protect the smaller decisions from being swept away with the larger ones.

The five principles that govern everything

The Mental Capacity Act code of practice, published by the government, sets out five principles that apply to every decision made under the Act.

  • A person must be assumed to have capacity unless it is established that they lack 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.

  • A person is not to be treated as unable to make a decision merely because they make an unwise one.

  • Anything done for, or on behalf of, a person who lacks capacity must be done in their best interests.

  • Before doing it, 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.

The third principle is the one that causes most friction inside families. A decision that looks reckless — refusing a treatment, giving money away, staying in a house that is hard to manage — is not evidence of incapacity. Adults are entitled to make choices their children disagree with.

The test itself

Assessing capacity involves two stages. The first asks whether the person has an impairment of, or a disturbance in the functioning of, the mind or brain. The second asks whether that impairment means they are unable to make the specific decision in question.

A person is unable to make a decision if they cannot do any one of four things: understand the information relevant to it, retain that information, use or weigh it as part of the process of deciding, or communicate their decision by any means at all. Those elements are set out in sections 2 and 3 of the Mental Capacity Act 2005.

Retention is often misunderstood. The Act is explicit that being able to hold information only for a short period does not, by itself, mean a person lacks capacity.

Who does the assessing

There is no single official assessor. In everyday life, the person who has to make the assessment is the person proposing to act: the bank considering a transaction, the social worker arranging care, the clinician proposing treatment, the attorney about to sign. For significant decisions, particularly medical ones or those likely to be challenged, a professional assessment is usual — but the Act does not require a doctor's certificate before ordinary decisions can be taken.

Helping the person decide comes first. That means information in plain language, at the right time of day, in a familiar place, with hearing aids and glasses to hand, with an interpreter or communication aid where needed, and with more than one attempt.

Best interests is not the same as best value

Where a person genuinely cannot make a decision, whoever acts must act in their best interests. GOV.UK guidance on making decisions for someone else makes clear this is a structured judgement, not a personal preference.

It requires the decision-maker to consider the person's past and present wishes and feelings, their beliefs and values, and anything else they would be likely to consider if able to do so — and to consult family, carers, attorneys and deputies where it is practical and appropriate. A best interests decision is an attempt to reach the decision that this person would recognise as theirs, not the cheapest or most convenient outcome for everyone else.

The judgement also cannot be made on the basis of age, appearance, or an assumption drawn from a condition or behaviour.

Planning ahead beats assessing later

Two documents reduce how often any of this has to be argued about. A lasting power of attorney appoints someone the person actually trusts, and can be made at any point while capacity remains. An advance decision to refuse treatment records a refusal of specific treatment in advance, and where valid and applicable it must be followed.

Where no such document exists and a serious decision needs to be made — selling a home, managing a pension, settling where someone lives — the route is an application to the Court of Protection.

Different rules elsewhere in the UK

The Mental Capacity Act 2005 applies in England and Wales only. Scotland has its own adults with incapacity framework, overseen by the Office of the Public Guardian (Scotland), with its own guardianship and intervention orders. Northern Ireland operates a separate framework, and nidirect explains the enduring power of attorney route used there.

If you need advice about a specific decision, the Law Society of England and Wales maintains a public find a solicitor service, and Citizens Advice publishes free guidance on managing affairs for someone else.

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