The Care CompassMental Capacity Explained: What Every Family Should Know
You'll see the phrase "mental capacity" everywhere in later-life care. On LPA forms, in conversations with GPs, in social services assessments. It's usually mentioned in passing, as though everyone already knows what it means. Most families don't, until they're suddenly the ones trying to work it out.
This guide explains what capacity actually means under UK law, how it's assessed, and why it matters. Whether you're setting up a Lasting Power of Attorney, supporting someone through a diagnosis, or trying to understand a decision that's already been made.
The law behind the word
In England and Wales, mental capacity is governed by the Mental Capacity Act 2005. It applies to everyone aged 16 and over, and it exists to protect two groups at once: people who may struggle to make certain decisions, and people who want to plan ahead while they still can.
The Act rests on five principles, and they're worth reading properly. They shape every capacity-related decision that follows:
1. Assume capacity. Every adult is presumed to have the capacity to make their own decisions, unless it's proved otherwise.
2. Support the person first. Someone must be given all practical help to make their own decision before anyone concludes they can't. Simpler language, more time, a better format, a calmer moment.
3. Unwise isn't the same as incapable. A person is allowed to make a decision others disagree with, or think is a bad idea. That alone doesn't mean they lack capacity.
4. Best interests. Anything done or decided on behalf of someone who lacks capacity must be done in their best interests.
5. Least restrictive option. Before acting on someone's behalf, the option chosen should be the one that interferes least with their rights and freedom.
How capacity is actually assessed
Capacity isn't a single label attached to a person. It's assessed for each specific decision, at the specific time it needs to be made. Someone can have capacity to decide what to have for lunch but lack capacity, on the same day, to decide whether to sell their house. The two aren't linked.
The assessment itself generally follows two stages:
Stage one, is there an impairment? Is there evidence of an impairment or disturbance affecting how the person's mind or brain works. For example, dementia, a brain injury, or a mental health condition?
Stage two. Does it affect this decision? Because of that impairment, is the person unable to do one or more of the following: understand the information relevant to the decision, retain that information long enough to use it, weigh it up as part of the decision-making process, or communicate their decision by any means?
A GP, social worker, or solicitor can carry out this assessment. It doesn't always require a specialist. Professionals across health and social care are expected to be able to assess capacity as part of their everyday practice.
Capacity can fluctuate, and that matters
This is one of the most important things families misunderstand: a diagnosis like dementia does not automatically mean someone has lost capacity. Capacity can fluctuate. Better in the morning than the evening, clearer on some days than others. Someone can still have capacity for many decisions long after a diagnosis, and it's the specific decision being assessed, not the diagnosis itself, that determines the answer.
This is exactly why timing matters so much with something like a Lasting Power of Attorney. It must be signed while the donor still has capacity to understand what they're agreeing to. Waiting until a diagnosis has progressed further can mean the window has already closed.
What happens if someone lacks capacity
If it's assessed that someone lacks capacity for a specific decision, whoever is acting for them, an attorney, a family member, or a professional, must make that decision in the person's best interests, taking into account what matters to them, their past wishes, and the views of people who know them well.
Where someone lacking capacity is in a care home or hospital and their care arrangements restrict their freedom in ways that amount to a deprivation of liberty, additional legal safeguards apply, currently the Deprivation of Liberty Safeguards (DoLS). A replacement system, the Liberty Protection Safeguards, has been planned for several years but still hasn't come into force as of today. DoLS remains the system actually in use.
Why this matters before you ever fill in a form
Understanding capacity properly changes how you approach almost everything else in this journey. It's why an LPA has to be set up early, why a diagnosis alone doesn't remove someone's voice in their own care, and why "unwise" decisions by someone you love aren't automatically a sign that something needs to be taken over.
A word from The Care Compass
When my mum's dementia was advancing, one of the hardest things wasn't the diagnosis itself. It was not understanding what she could and couldn't still decide for herself, and feeling like we were guessing. Learning that capacity is assessed decision by decision, not as one fixed switch, would have saved us a lot of unnecessary worry and a few conversations we didn't need to have as early as we did.
This article is for guidance only and does not constitute legal or medical advice. If you have concerns about someone's capacity, speak to their GP or a solicitor experienced in this area.
You don’t have to navigate it alone.