Author and Publication
Author: Nellie Supports Ltd
Publication Date: 15/05/2026
Citation
39 Essex Chambers, Mental Capacity Guidance Note: Carrying out and recording capacity assessments.
Copywright
Copyright © 2026 Nellie Supports Ltd. All rights reserved.
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Mental Capacity Act 2005: Key Principles Explained
The Mental Capacity Act 2005 rests on five principles: a presumption of capacity; the right to all practicable support before anyone concludes capacity is lacking; the right to make unwise decisions; best interests as the basis for any decision made on behalf of a person who lacks capacity; and the least restrictive option. Every capacity assessment and every best interests decision in England and Wales is supposed to run on these five rails, and most failures in practice are a failure of one of them. This guide explains each principle in plain English and shows how they appear in a properly conducted assessment.
The five principles at the heart of the Act
Section 1 of the Mental Capacity Act 2005 sets out five principles that govern all decisions made under it. They are the presumption of capacity, the duty to support decision-making, the right to make unwise decisions, the best interests duty, and the least restrictive principle. Every assessment and decision must be read through these principles.
The presumption of capacity
A person must be assumed to have capacity unless it is established that they lack it. The burden is on the person asserting incapacity, and the standard is the balance of probabilities. This means capacity is the starting point, and an assessment exists to test that presumption, not to confirm an assumption that someone cannot decide.
Supporting the person to decide
A person is not to be treated as unable to make a decision unless all practicable steps to help them have been taken without success. This includes giving information in an accessible way, choosing the right time and place, using communication aids, and involving people who can help. Support must come before any conclusion about capacity.
The right to make unwise decisions
A person is not to be treated as lacking capacity merely because they make a decision others consider unwise. Capacitous adults are entitled to make choices that seem eccentric, risky or wrong to others. The assessment must distinguish a genuinely unwise but capacitous decision from an inability to use or weigh the relevant information.
Acting in a person's best interests
Any act or decision taken for someone who lacks capacity must be in their best interests. This is a structured process under section 4, taking account of the person's past and present wishes, feelings, beliefs and values, and the views of those close to them. Best interests is not simply what others think is best for the person.
The least restrictive option
Before any act is done for a person who lacks capacity, regard must be had to whether the same outcome can be achieved in a way that is less restrictive of their rights and freedom of action. Where two options would meet the need, the one that interferes least with the person's liberty should be preferred.
How the principles work together
The principles are not a menu to choose from. A decision must presume capacity, offer support, respect unwise choices, and, only where capacity is genuinely lacking, move to a best interests decision taken in the least restrictive way. Skipping any step, particularly the duty to support, undermines the lawfulness of what follows.
Why the principles matter in assessments
A capacity assessment that ignores the principles is open to challenge. Failing to provide support, treating an unwise decision as proof of incapacity, or moving to best interests without establishing a lack of capacity are all errors that the principles are designed to prevent. A sound assessment shows the principles being applied.
Key takeaway
The five principles protect autonomy while providing a lawful route to act for those who genuinely cannot decide. They require capacity to be presumed, decision-making to be supported, unwise choices to be respected, and any intervention to be in the person's best interests and as unrestrictive as possible.
Frequently asked questions
What are the five principles in short?
Presume capacity; support decision-making before concluding it is absent; an unwise decision is not an incapable one; decisions for a person who lacks capacity must be in their best interests; and choose the least restrictive option. Every good assessment shows all five at work.
Does the Act apply to everyone over sixteen?
The Act applies to people aged 16 and over in England and Wales, though some things it governs, such as making a will or granting a Lasting Power of Attorney, additionally require the person to be 18.
What is the difference between capacity and best interests?
Capacity is the gateway question: can this person make this decision? Best interests only arises after a proper finding that they cannot. Running the two together is one of the commonest errors in practice, because no decision can be made for a person in their best interests while they retain capacity to make it themselves.
Related mental capacity assessment pages
These internal links help readers move from this guide to the most relevant Nellie Supports service page, assessment option or legal framework page.
Want the principles applied properly?
Nellie Supports provides independent, decision-specific mental capacity assessments across England and Wales through a permanent employed team of registered professionals, with a same working day response to every enquiry and every report peer reviewed before delivery. Call 0333 987 5118 or visit the mental capacity assessment service page.