The core answer
A parent who is starting to lose their memory still has full legal capacity until a court declares them incompetent or quasi-incompetent. A doctor's diagnosis does not make every document void. What decides each document is the person's understanding at the moment of signing, and that has to be proven with evidence.
The early stage is the best time to act: make a will in the proper form, sign a power of attorney for specific tasks, list the assets and record that the parent understood what they signed. A power of attorney cannot be a long-term plan, because an agency contract ends when the principal dies or becomes incompetent (section 826).
Once the parent can no longer manage money and property, the stable route is for a relative with standing to ask the court to declare the parent incompetent and appoint a guardian under section 28. For milder cases the court may declare quasi-incompetence under section 32. The person can still make ordinary transactions but needs the curator's consent for certain important ones.
1. How does a medical diagnosis differ from legal capacity?
| Point | Medical diagnosis | Legal status |
|---|---|---|
| Who decides | A doctor, from symptoms and tests | A court, by an order under section 28 or 32 |
| Effect on documents | Evidence of mental state | The order changes the person's status and is published in the Royal Gazette |
| Levels | Many stages; symptoms fluctuate | Two statuses: incompetent and quasi-incompetent; everyone else is treated as having capacity |
| What the court examines | Test results, treatment history | Understanding of the act at the time of signing |
Some patients have lucid and confused periods. A document signed in a lucid period may be valid even though the person is ill. A document signed during a bad period may be challenged even without a court order. The question to ask each time is "what did they understand on the day they signed", not "what illness do they have".
2. Before any court order, when can the parent's acts be set aside?
Section 30 sets two conditions. An act of a person of unsound mind whom the court has not declared incompetent is voidable only if
- it was done while the person was of unsound mind, and
- the other party also knew the person was of unsound mind.
Both are required. In practice:
- It is not enough to show the parent was confused. You must also show the other party knew, for example a buyer, transferee, bank or relative present at the time.
- If the other party neither knew nor had reason to know, the act may stand even if the parent was truly of unsound mind.
- If a child is the other party and lives close to the parent, a claim of "I did not know about the memory loss" is hard to sustain.
A voidable act is valid until it is annulled; it is not automatically void. Annulment has its own persons entitled and time limits, so check before acting.
3. How do incompetence and quasi-incompetence differ, and who can apply?
| Point | Incompetent person | Quasi-incompetent person |
|---|---|---|
| Provision | Section 28 | Section 32 |
| Ground | Unsound mind | Physical disability, weak or defective mind, habitual prodigality, addiction to alcohol or drugs or similar causes, to the extent the person cannot manage affairs alone or manages them in a way that may harm their own or the family's property |
| Supervisor | Guardian | Curator |
| Effect on acts | Acts done personally are voidable (section 29) | Can act alone, but the acts listed in section 34, such as investing, borrowing, gifts, acquiring or releasing rights in immovable property, compromising disputes, need the curator's prior consent; breach makes them voidable |
| Publication | Royal Gazette | Royal Gazette |
Section 28 allows the following to apply: the spouse, ascendants (parents, grandparents, great-grandparents), descendants (children, grandchildren and further), a guardian or curator, the person who cares for the individual, and the public prosecutor. Section 32 allows the same group.
The petition usually needs a medical certificate or test results, treatment records, witnesses who know the patient and information about the assets to be managed. The court may hold a hearing on the person's condition and check that the proposed guardian is suitable. Filing details, the competent court and timing depend on each case.
4. What can a guardian do, and when is court permission needed?
A guardian acts for the person declared incompetent: pays for care, manages property and makes transactions within the limits set by law and the court order.
The power is not unlimited. Section 1598/18 applies the rules on the rights and duties of guardians by analogy where the guardian is someone other than a parent or spouse (for example a child caring for the parent), and section 1598/3 makes section 1574 apply by analogy. Section 1574 lists property transactions that need court permission, such as
- selling, exchanging, selling with right of redemption, mortgaging or releasing a mortgage over immovable property or mortgageable movables
- leasing immovable property for more than three years
- lending money
- gifts, except those reasonable for charity, social occasions or moral duty
- guaranteeing debts in a way that may burden the person
- compromising a dispute or sending it to arbitration
Applying these sections to an adult under guardianship is by analogy, so a guardian should not decide alone what is allowed. Read the appointment order and take advice before selling land, mortgaging a house or transferring assets. Acts done without permission may be set aside and the guardian may be held liable or removed.
5. How far does a power of attorney go, and when does it end?
A power of attorney lets a child carry out transactions for the parent, such as withdrawing money for medical bills, renewing a lease or dealing with agencies. It has limits.
- It ends when the principal becomes incompetent. Section 826 ends an agency contract when either party dies, becomes incompetent or goes bankrupt, unless that conflicts with the contract or the nature of the matter. The wording is "becomes incompetent", meaning a status ordered by a court. Serious illness without a court order does not by itself end the agency under this section, but that is not a safe position: the principal needs understanding on the day of signing, and a third party may refuse a document that looks suspicious.
- The agent is limited to the stated scope. Write it task by task, for example withdrawals from this account up to an agreed limit. Avoid "do everything on my behalf" unless it is truly needed.
- It can be revoked at any time. The principal may still revoke the agent (section 826, first paragraph) if they change their mind or conflict arises.
- The agent should report and keep funds separate. Use the parent's money for the parent and keep receipts.
In short, a power of attorney fits the early stage and specific transactions. Once the situation reaches the point of asking a court to appoint a guardian, it is no longer the main tool.
6. Is a will made once symptoms begin valid?
Section 1704 separates two cases.
- A person the court has declared incompetent makes a will. The will is void.
- A person alleged to be of unsound mind, but not yet declared incompetent, makes a will. The will is invalid only if it is proven that the person was of unsound mind when making it.
So a will made before any court order is not invalid merely because the person has dementia. Someone must prove that on that day the person was of unsound mind. The burden falls on the party wanting the will set aside. A will made after the court order is void outright.
Apart from mental state, the will must follow a legal form. Common forms:
- Ordinary witnessed will (section 1656): in writing, dated, signed by the testator in front of at least two witnesses present together, who sign to certify the signature.
- Holograph will (section 1657): the testator writes the whole text, the date and the signature by hand.
- Public document will (section 1658): the testator states the content to the district official in front of at least two witnesses; the official records and reads it back, all sign, and the official signs, dates and stamps it.
For a patient whose mental state may be challenged later, the district-office form has the advantage that an official and witnesses are part of the process. It does not remove every dispute about capacity but it helps ensure there are people who saw the event. See the guide on contested wills linked below for collecting evidence.
7. What evidence should be gathered on signing day?
These steps are not legal requirements, but they help prove later that the parent understood.
- A doctor's note or certificate on the same day. Ask a doctor who knows the patient to record that on that day they knew the date, knew what they were doing and knew their main assets.
- Independent witnesses. Choose people who do not benefit under the will, such as a neighbour or an official, not a child who receives a larger share.
- Let the parent explain in their own words how the assets will be divided and why. Record what they say, not what a child says for them.
- Video. Record without cuts and ask open questions: what assets do you have, how many children, who gets what. A video is supporting evidence, not the will itself (see the guide on digital wills).
- Keep beneficiaries out of the room while the parent states their wishes.
- Choose the best time of day, for example the morning, if symptoms worsen in the evening.
- Keep the original and copies in several places and note who knows where they are.
8. Options by stage
| Stage | Features | Options | Cautions |
|---|---|---|---|
| Early | Still decides on money and property; some forgetfulness | A will in proper form with the evidence in section 7; a power of attorney for specific tasks; a list of assets and accounts; an executor named in the will; advice on joint accounts or joint management | A joint account does not automatically make the money the child's, and earlier transfers to one child can be challenged by siblings |
| Moderate | Confused at times; struggles to manage money | Assess whether to ask the court for quasi-incompetence or incompetence; sign key documents only in lucid periods with evidence | Documents from this period are the most likely to be challenged; gather strong evidence |
| Advanced | Cannot recognise people or money; cannot decide on property | Ask the court to declare incompetence and appoint a guardian under section 28 | Nobody should sign for the parent or guide their hand; a power of attorney signed when mental state was poor risks being overturned |
| Sibling dispute | Disagreement over who cares or who is guardian | Ask the court to consider a suitable guardian; transparent income and expense records; mediation | The court looks at suitability and the patient's interests, not birth order |
9. Banks and the land office
Banks have their own internal rules for accounts of elderly customers with mental-state concerns. Some may ask for further documents, such as a court order appointing a guardian, or refuse a power of attorney that looks irregular. Ask the branch holding the account what it requires before the day you need it.
A land office may also check the document showing the representative's authority. For a transfer of property of a person declared incompetent, the guardian may need to show the appointment order and a court permission order depending on the type of transaction. Ask the officials before booking a transfer date, since practice may differ a little between offices.
Prepare copies of the ID of the patient and the person acting, the house registration, land title documents, bank books and proof of the relationship.
10. Siblings in conflict, and transparency
What takes families to court is often not the illness but distrust about money.
- Agree on a main carer and tell all siblings before filing anything.
- Keep an account of income and spending. The parent's money is for the parent: treatment, nursing, food. Keep receipts and send a monthly summary to siblings.
- Keep accounts separate from personal money.
- Do not transfer assets to yourself on the claim that the parent gave them to you. Without clear evidence and notice to siblings, suspicion follows at once.
- If you suspect a sibling is taking advantage, gather evidence, check the bank books, and consider asking the court to appoint a neutral guardian or to remove an unsuitable one.
11. Common mistakes
- Assuming that a diagnosis makes all documents void. Look at the state of mind on the signing day and the conditions in sections 30 and 1704.
- Assuming the parent can handle everything until the last day, and doing nothing until a court filing is needed in an emergency.
- Using a power of attorney as a long-term plan, when the agency ends if the principal becomes incompetent.
- Letting the child who benefits under the will take the parent to make it and sit in the room.
- Having the patient sign every document without explaining the content.
- A guardian selling or mortgaging property without checking whether court permission is needed.
- Mixing the parent's money with one's own and not keeping receipts.
- Making several new wills as symptoms worsen, so later ones conflict with the first and invite disputes.
12. Checklist
- Note symptoms, dates and treatment; get current test results or a medical certificate
- List assets, bank accounts, land, debts, insurance policies and where title documents are kept
- Assess how well the parent understands their assets and will right now
- If making a will, choose the form (section 1656, 1657 or 1658) with a same-day medical note, independent witnesses and a supporting video
- If signing a power of attorney, state the tasks and limits and require the agent to keep receipts
- Tell all siblings about the plan and who is the carer
- Ask the bank and land office what documents they require
- If the parent can no longer manage money, take advice on asking the court to declare incompetence or quasi-incompetence
- Once a guardian or curator is appointed, read the order and seek court permission before any major transaction
Further reading
- Choosing Your Executor in a Thai Will: How Many, Who, and a Backup — when planning the will in advance
- A Will on a Phone, in a PDF, a LINE Message or a Video: Does It Count in Thailand? — how far a video helps as evidence
- Suspect a Will Is Forged or Was Made Under Pressure? What Heirs Should Gather and How Long They Have — when siblings challenge a will
- Parents Pass Away – How Is the Inheritance Divided: A Guide for Heirs and Estate Administrators — the steps after a parent dies
- A Minor Child Inherits: How Far Can a Parent Manage the Money and Sell Property, and When Is Court Permission Needed? — the section 1574 rule applied to guardians
A parent who is starting to lose their memory keeps legal capacity until a court orders otherwise, and a diagnosis alone does not void documents. Make the will and gather the evidence in the early stage, and prepare to apply to the court once the parent can no longer manage property.
We help assess your family's situation, plan wills and powers of attorney, prepare signing-day evidence, file petitions asking the court to declare incompetence or quasi-incompetence, and assist when siblings are in dispute. For an initial consultation, call +66 92 254 2045 or send us the details. See also our estate and will practice and family practice.
This guide is published by Suwanvara Law Firm — a Khon Kaen law firm established in 1986. General information only, not legal advice on a specific matter.
