Self Assessment: register by 5 October or face a penalty

Who has to register

Anyone who needs to file a Self Assessment return for the 2025-26 tax year for the first time must register with HMRC by 5 October 2026. Missing this deadline can trigger a failure to notify penalty calculated as a percentage of the tax owed, separate from late filing penalties.

According to HMRC's guidance on registering for Self Assessment, you must register if you have income that is not already taxed at source. The specific triggers listed by HMRC include being self-employed with gross income above £1,000, receiving untaxed income such as rental income or income from savings and investments, or having certain dividend income that exceeds the tax-free allowance. Additionally, you may need to register if you are liable to the High Income Child Benefit Charge, which applies when your adjusted net income is above £50,000 and you or your partner claim Child Benefit.

HMRC also states that you must register if you have not received a notice to file a return but your income falls into one of these categories. The registration requirement applies to individuals, not to companies or trusts, and it is separate from the annual filing obligation. For the 2025-26 tax year, the registration deadline is fixed at 5 October 2026, regardless of when your income first arose during that tax year. If you are unsure whether you need to register, HMRC's online tool can help determine your obligations, but the deadline remains the same.

It is important to note that registration is not required if you are already in the Self Assessment system from a previous year, as you will receive a notice to file. However, if you have never filed a return and your income triggers a requirement, you must register by the deadline. HMRC's guidance emphasises that the registration process is separate from filing the return itself, and failing to register does not remove your obligation to file later.

How registration works

Registration for Self Assessment is done online through HMRC's digital service. For the self-employed, the process creates a record for your business and issues a Unique Taxpayer Reference (UTR), a 10-digit number used for all future tax correspondence. Other filers, such as those with untaxed income, follow a similar but distinct registration path.

According to HMRC's guidance, self-employed individuals must register for Self Assessment and also register for Class 2 National Insurance if their profits are above the Small Profits Threshold. The registration process asks for personal details, business start date, and contact information. Once registered, HMRC issues a UTR by post within 10 working days, though this can take longer during peak periods. The UTR is essential for filing your return online, as you must quote it when submitting your tax return.

For individuals who need to register due to untaxed income, such as rental income or savings interest, the process is similar but does not require business details. HMRC's online registration service will ask for your National Insurance number and details of the income source. If you are registering because of the High Income Child Benefit Charge, you must also provide your partner's details and the amount of Child Benefit claimed. In all cases, registration is free of charge and can be completed at any time before the deadline.

Once you have a UTR, you can use it to file your return online, either through HMRC's own service or via commercial software. The UTR remains with you for life, even if you stop being self-employed or no longer need to file returns. HMRC advises that you keep your UTR safe and quote it in all correspondence. If you lose your UTR, you can request it again through your online account or by contacting HMRC, but this may take time, so it is best to keep a record.

Failure to notify penalties

If you miss the 5 October registration deadline, HMRC can charge a failure to notify penalty. This penalty is calculated as a percentage of the tax you owe, not a flat fee, and the percentage increases with the delay. The penalty can be reduced if you have a reasonable excuse.

According to HMRC's guidance on penalties for failure to notify, the penalty is based on the amount of tax that was unpaid as a result of not notifying HMRC. The percentage charged depends on how long the failure continues. For example, if you notify HMRC within 12 months of the tax being due, the penalty is up to 30% of the tax owed. If the delay is between 12 and 24 months, the penalty rises to up to 70%, and if it exceeds 24 months, it can reach 100% of the tax due. These percentages are set out in HMRC's penalty tables.

HMRC states that the penalty is separate from any late filing or late payment penalties you may incur. Even if you eventually file your return and pay the tax, the failure to notify penalty still applies because it addresses the initial failure to register. The penalty is calculated on the tax that would have been due for the period, not on the total income. For example, if you owed £1,000 in tax and you delayed notification by 10 months, the penalty could be up to £300.

However, HMRC allows for a reduction in the penalty if you have a reasonable excuse, such as a serious illness or events beyond your control. You must tell HMRC about the excuse as soon as possible after the failure. HMRC also offers a reduction if you make a prompted or unprompted disclosure. If you contact HMRC before they contact you, the penalty may be reduced by up to 30% for the disclosure, but this is at HMRC's discretion. The exact reduction depends on the circumstances, as detailed in HMRC's penalty guidance.

The full deadline sequence

For the 2025-26 tax year, the key deadlines are: register by 5 October 2026, file paper returns by 31 October 2026, file online by 31 January 2027, and pay any tax due by 31 January 2027. Missing any of these dates can result in separate penalties.

According to HMRC's guidance on Self Assessment tax return deadlines, the registration deadline for first-time filers is 5 October 2026. After registration, you must file your return by the appropriate deadline. If you choose to file a paper return, the deadline is 31 October 2026. For online filing, the deadline is 31 January 2027. These dates are fixed and do not change, even if the 5 October deadline falls on a weekend or bank holiday, in which case HMRC usually accepts submissions on the next working day, but this is not guaranteed.

In addition to filing, you must pay any tax you owe by 31 January 2027. This includes the balancing payment for the 2025-26 tax year and the first payment on account for the 2026-27 tax year, if applicable. If you file online, you can choose to pay by direct debit, which may be set up after the filing deadline, but the payment must still reach HMRC by 31 January. If you miss the payment deadline, you will be charged interest on the outstanding amount, as well as a late payment penalty of 5% of the tax due if it remains unpaid after 30 days.

HMRC also notes that if you file online, you can request a payment plan if you cannot pay in full, but this must be done before the deadline to avoid penalties. The sequence of deadlines is designed to give you time to register, file, and pay, but each step has its own consequences for missing it. For example, filing late incurs a £100 penalty if your return is up to 3 months late, and further penalties are added after that, as outlined in HMRC's deadlines guidance.

What to do if the deadline has been missed

If you have already missed the 5 October 2026 registration deadline, you should register as soon as possible. HMRC's guidance states that the failure to notify penalty is calculated based on the delay, so acting quickly can minimise the penalty. You may also need to file your return and pay any tax due to avoid further penalties.

According to HMRC, you can still register for Self Assessment after the deadline, but you must do so without delay. The registration process is the same, and you will receive a UTR, but you must then file your return by the relevant deadline, which may already have passed. If you miss the online filing deadline of 31 January 2027, you will incur a late filing penalty of £100, as stated in HMRC's deadlines guidance. This penalty applies even if you have no tax to pay.

If you have not yet filed your return, you should do so immediately, even if you cannot pay the tax. HMRC advises that filing late is better than not filing at all, as it can reduce the penalties you face. You can also contact HMRC's Self Assessment helpline to discuss your situation. HMRC may agree to a payment plan if you cannot pay the tax in full, but you must contact them before the payment deadline to arrange this. If you have a reasonable excuse for missing the registration deadline, you can appeal against the failure to notify penalty, but you must provide evidence to support your claim.

In summary, the best course of action is to register, file, and pay as soon as possible. HMRC's penalty guidance makes clear that the longer you delay, the higher the penalties become. If you are unsure about your obligations, you can seek advice from a tax professional, but you should not wait, as the penalties are calculated on a daily basis after the initial deadline.

Frequently asked questions

What is the deadline for registering for Self Assessment for the 2025-26 tax year?

Anyone who needs to file a Self Assessment return for the 2025-26 tax year for the first time must register with HMRC by 5 October 2026. This deadline applies to individuals who have not previously filed a return but now meet the criteria for self assessment, such as receiving untaxed income or capital gains above certain thresholds.

What happens if a person misses the Self Assessment registration deadline?

Missing the 5 October 2026 registration deadline can trigger a failure to notify penalty. This penalty is calculated as a percentage of the tax owed for the 2025-26 tax year. The percentage depends on the reason for the failure, ranging from lower penalties for careless mistakes to higher penalties for deliberate concealment. This penalty is separate from any late filing penalties.

Is the failure to notify penalty separate from late filing penalties?

Yes, the failure to notify penalty is separate from late filing penalties. Late filing penalties apply when a tax return is submitted after the 31 January filing deadline. The failure to notify penalty applies specifically for not registering for Self Assessment by the 5 October deadline. A person can potentially face both penalties if they miss both deadlines.

How is the failure to notify penalty calculated?

The failure to notify penalty is calculated as a percentage of the tax owed for the 2025-26 tax year. The percentage varies based on the behaviour of the taxpayer. For a careless mistake, the penalty is up to 30% of the tax due. For a deliberate but not concealed error, it is up to 70%. For deliberate and concealed errors, the penalty can reach 100% of the tax owed.

Can a person avoid the failure to notify penalty if they have a reasonable excuse?

HMRC may consider a reasonable excuse as a defence against a failure to notify penalty. Examples of reasonable excuses include serious illness, the death of a close relative, or circumstances beyond the person's control. The excuse must be genuine and the person must have acted promptly to remedy the failure once the excuse ended. Each case is judged on its own facts.