Who decides if someone has capacity?

By Matt Cresswell, Founder at Power of Attorney Online and author of
Everything you ever wanted to know about LPAs (but were afraid to ask)

18th August 2025

Updated: Friday 9th January 2026

Who decides if someone has capacity? Mental capacity is central to creating a valid Lasting Power of Attorney. If a person does not have capacity at the time an LPA is made, the document will not be legally valid. Understanding how capacity works, and why timing matters, is essential for anyone planning ahead or supporting a loved one.

What is mental capacity under UK law?

Mental capacity is a person’s ability to make a specific decision at the time it needs to be made. Under the Mental Capacity Act 2005, every adult is assumed to have capacity unless it is proven otherwise.

A person lacks capacity for a particular decision if they cannot understand the relevant information, retain it long enough to make the decision, weigh up the options or communicate their choice in any way.

Capacity is not about intelligence or diagnosis. Someone can have a medical condition and still have capacity.

Is mental capacity always clear cut?

No. Mental capacity is decision specific and time specific. A person may be able to make simple everyday choices but struggle with complex financial or legal decisions.

Capacity can also fluctuate. Conditions such as dementia, brain injury, mental illness or the effects of medication can cause good days and bad days. This creates grey areas where families may disagree or feel unsure about whether someone still has capacity.

Why does mental capacity matter when making an LPA?

An LPA must be made while the donor fully understands what they are signing and the powers they are giving to their attorneys. This is why every LPA includes a certificate provider.

The certificate provider confirms that the donor understands the document, is making it freely and is not under pressure. If there is any doubt about capacity at the time of signing, the LPA can be challenged or rejected.

If capacity is lost before an LPA is completed, it is no longer possible to make one.

What happens if capacity is lost before an LPA is in place?

If someone loses capacity without an LPA, their family cannot step in automatically. Instead, they must apply to the Court of Protection for a deputyship order.

This process is slower, more expensive and more restrictive than an LPA. The court decides who can act, what decisions they can make, and supervises them on an ongoing basis.

Do you need a professional capacity assessment to make an LPA?

In most cases, no. A formal medical assessment is not usually required to create an LPA. However, if capacity may later be questioned, early action is crucial.

Where there is any doubt, taking advice can help reduce the risk of future challenges.

What is the key message about mental capacity and LPAs?

Mental capacity is complex, personal and often misunderstood. Waiting until capacity is in question is one of the most common reasons families run into serious legal difficulties.

The safest approach is to plan early, while capacity is clear. Creating a Lasting Power of Attorney in good time, using a service like Power of Attorney Online, ensures your wishes are respected and removes uncertainty for the people who care about you.

Taking action now protects your future and gives everyone peace of mind.

Written by Matt Cresswell

Matt Cresswell is the Founder of Power of Attorney Online, a service helping people across England and Wales create Lasting Powers of Attorney through guided online support and expert document reviews. He founded the business following his father's diagnosis with dementia, after experiencing first-hand how confusing the LPA process can be for families. Matt's expertise focuses on Lasting Powers of Attorney, planning ahead and helping families prepare for the future before a crisis occurs.