The Mental Capacity Act 2005 – Legal Framework, Tests, and Expert Application
What Is the Mental Capacity Act 2005?
Legal framework, the capacity test, and how assessments should be approached
The Mental Capacity Act 2005 (MCA) provides the main legal framework in England and Wales for supporting adults (aged 16+) to make their own decisions, and for making decisions only when necessary on behalf of someone who lacks capacity for a specific decision at a specific time.
It covers everyday decisions as well as major decisions about health, welfare, and property/finances, and it underpins mechanisms such as Lasting Powers of Attorney (LPAs), deputyship, advance decisions to refuse treatment, and applications to the Court of Protection.
The MCA is designed to promote autonomy and protect people from harm, without treating people as incapable just because they have a diagnosis, disability, or make a choice others disagree with.
The Five Statutory Principles of the MCA
The MCA is grounded in five statutory principles set out in Section 1 of the Act. These principles govern all decisions and acts taken under the MCA and must be followed without exception:
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Presumption of capacity
Every adult is presumed to have capacity unless it is established that they lack capacity for the specific decision. -
Support to make decisions
A person is not treated as unable to decide unless all practicable steps to support decision-making have been tried without success. -
Unwise decisions
Someone is not to be treated as lacking capacity only because they make a decision others see as unwise. -
Best interests
If a person lacks capacity for the decision, any act/decision on their behalf must be in their best interests. -
Least restrictive option
Before acting, decision-makers must consider whether the aim can be achieved in a way that is less restrictive of the person’s rights and freedom.
These principles are central to lawful practice and appear throughout the MCA guidance for assessors and decision-makers.
The current legal test for capacity (Functional-First approach)
Capacity is always decision-specific and time-specific. A person may be able to make some decisions but not others, and capacity may fluctuate.
Step 1: Functional Test (the “can they decide?” question)
A person is unable to make the decision if they cannot do one or more of the following:
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Understand the relevant information
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Retain that information long enough to decide
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Use or weigh that information as part of the process of deciding
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Communicate their decision (by any means)
Step 2: Causative Test (the “because of what?” question)
If there are difficulties on the functional elements above, the key legal question is whether those difficulties are because of an impairment of, or disturbance in, the functioning of the mind or brain (for example dementia, delirium, brain injury, learning disability, severe mental illness, intoxication, etc.).
Both parts matter: it is not enough to identify a diagnosis; and it is not enough to show someone is making a risky choice. The issue is whether the person can complete the decision-making steps because of the impairment/disturbance.
What is “relevant information” for the decision?
The “relevant information” is the information the person needs to be able to understand, retain, and use/weigh for that particular decision, including the reasonably foreseeable consequences of deciding one way or another, or of making no decision.
(For many common decision-types—care, residence, contact, sex, litigation capacity, etc.—courts have given guidance on what counts as relevant information, and it must be tailored to the person’s situation.)
Capacity is not “global” and must not be based on assumptions
Under the MCA, you must not decide someone lacks capacity simply because of:
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age, appearance, condition, diagnosis, or behaviour; or
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because others think their decision is unwise.
Good practice also requires documenting what support was offered (communication aids, simplified explanations, timing/location, interpreters, advocates, etc.), because capacity can’t lawfully be concluded without trying practicable steps to support the person first.
The Court of Protection
The Court of Protection deals with disputes and applications about:
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whether a person has capacity for a particular decision;
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best interests decisions where agreement cannot be reached;
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appointment of deputies (property/affairs and, more rarely, welfare);
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issues relating to LPAs (including validity, scope, and revocation);
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other court powers under the MCA.
When an application is made, evidence about capacity is commonly provided using COP3 – Assessment of capacity (01/24).
Decision-Specific and Time-Specific Capacity
Under the MCA, capacity is never a global label. It must be assessed in relation to a specific decision at a specific point in time. For example:
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A person may have capacity to decide where to live but lack capacity to consent to a complex financial transaction.
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Someone may lack capacity during an acute medical episode but regain capacity once the condition stabilises.
This principle ensures that decision-makers respect autonomy wherever possible and intervene only where necessary.
Expert Evidence and CPR Part 35 Compliance
When expert evidence is required in MCA matters - such as in litigation, deputyship disputes, or complex best interest determinations - expert witnesses must comply with Civil Procedure Rules (CPR) Part 35. This includes duties of impartiality, independence, and a clear exposition of reasoning grounded in evidence.
Expert reports should clearly articulate:
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The legal test applied,
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The factual basis of findings,
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How the statutory principles and two-stage test were satisfied or not,
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A reasoned explanation of any conclusions.
This distinguishes professional expert assessment from informal or undocumented capacity testing.
Key Legal Contexts for MCA Assessments
Mental capacity assessments are required in numerous legal and clinical contexts, including but not limited to:
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Lasting Powers of Attorney – capacity to grant or revoke LPA.
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Court of Protection (COP3) – formal assessments used in deputyship and litigation contexts.
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Testamentary Capacity – capacity to make a valid will.
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Property and Financial Decisions – capacity to manage finances, sell property, enter contracts.
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Healthcare and Personal Welfare Decisions – consent to treatment.
Each form of assessment must apply the same legal test, tailored to the demands of the particular decision.
The Mental Capacity Act 2005 is a foundational statute in England and Wales that codifies how decision-making capacity is understood, assessed, and acted upon. Its strength lies in balancing respect for autonomy with necessary safeguards where capacity is impaired.
Nellie Supports applies the MCA with strict legal precision, ensuring assessments are:
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Compliant with statutory principles,
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Grounded in rigorous application of the two-stage test,
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Fit for purpose in clinical, legal, and Court of Protection contexts.
Or explore our full Mental Capacity Assessment Service.
This page provides general information about the Mental Capacity Act 2005 in England and Wales and is not legal advice. Capacity assessments and best interests decisions are fact-specific. If you need advice about a particular situation, obtain legal advice or consult your organisation’s legal team. Last Reviewed [February, 2026}