What Happens If You Miss the Self Assessment Deadline?

01 Oct What Happens If You Miss the Self Assessment Deadline?

Missing a Self Assessment deadline can feel worrying, particularly when you are already busy running a business. However, the situation is usually manageable if you act promptly.

The key is to understand which deadline you missed:

• The deadline to register for Self Assessment
• The deadline to submit your tax return
• The deadline to pay your tax bill

These have different consequences. This guide explains what can happen if you file or pay late, when an appeal may succeed, and the practical steps to get back on track.

Important: This article covers the 2025/26 tax year, which ran from 6 April 2025 to 5 April 2026. Tax rules and HMRC interest rates can change, so check the latest HMRC Self Assessment guidance or speak to your accountant about your circumstances.

Which Self Assessment deadlines matter?

The first step is to identify exactly which deadline applies to you.

For most sole traders and company directors who file online, the important dates are:

• 5 October 2026: deadline to register for Self Assessment if you need to file for the first time
• 31 October 2026: deadline for paper tax returns
• 31 January 2027: deadline for online tax returns
• 31 January 2027: deadline to pay your balancing tax payment and, where applicable, your first payment on account
• 31 July 2027: deadline for your second payment on account, if one is due

If you are reading this before 5 October 2026 and you have never filed a tax return before, do not overlook the registration date. You can register for Self Assessment through GOV.UK.

Being a limited company director does not automatically mean you must complete a personal Self Assessment return. However, HMRC may require one if, for example, you have untaxed income, dividends above the relevant allowance, property income, capital gains or other income that needs reporting. You must also file if HMRC sends you a notice to do so.

What happens if you miss the 5 October registration deadline?

Missing the registration date is not the same as filing a tax return late.

If you need to file for the first time for 2025/26, you should tell HMRC by 5 October 2026. This also applies if you registered previously but did not need to send a return for 2024/25.

If you register after 5 October:

• HMRC will usually send you a letter or email with a different filing deadline
• That deadline will generally be three months from the date of the letter or email
• The payment deadline does not move
• Any tax owed for 2025/26 will still normally be due by 31 January 2027

Missing 5 October does not automatically create the standard £100 late-filing penalty. However, HMRC says you may receive a failure to notify penalty if you register late and do not pay all the tax you owe by 31 January.

The sensible response is to register as soon as you realise you need to. Do not wait for HMRC to contact you.

What are the late-filing penalties?

The late-filing penalties increase the longer your return remains outstanding.

If you miss your paper or online filing deadline, HMRC can charge:

• £100 immediately once the return is late
• £10 per day after three months, up to 90 days, giving a maximum daily penalty of £900
• A further penalty after six months of 5% of the tax due or £300, whichever is greater
• Another penalty after 12 months of 5% of the tax due or £300, whichever is greater

The initial £100 penalty can apply even if:

• You have no tax to pay
• You have already paid your tax bill
• Your tax return shows a repayment due

These are penalties for filing late. They are separate from charges for paying late.

For example, suppose your online return is due by 31 January 2027 and you submit it in February. You could receive the £100 fixed penalty. If you still have not submitted it after three months, daily penalties may begin. Waiting longer can result in the percentage or minimum £300 penalties being added.

The best way to limit the impact is to submit the return as soon as possible, even if you cannot pay the full amount immediately.

What happens if you pay your tax late?

Late payment creates a separate set of penalties and interest.

For Self Assessment tax due by 31 January, HMRC can charge:

• 5% of the unpaid tax after 30 days
• A further 5% of the unpaid tax after six months
• Another 5% of the unpaid tax after 12 months
• Interest on the unpaid amount from the day after the payment deadline

These payment penalties can apply alongside late-filing penalties. Filing your return on time does not prevent payment penalties if the tax bill remains unpaid.

Equally, paying your estimated tax on time does not prevent a late-filing penalty if the return itself is submitted after the deadline.

Interest continues until the amount owed is paid. The applicable HMRC interest rate can change, so check the latest information when working out what you owe. You can use HMRC’s Self Assessment penalty calculator to estimate possible charges.

If you are struggling to pay, do not ignore the bill. You may be able to agree a payment arrangement with HMRC. Read the GOV.UK guidance if you cannot pay your tax bill on time, and contact HMRC as early as possible.

When will HMRC accept a reasonable excuse?

You may be able to appeal a penalty if an unexpected event genuinely prevented you from meeting the deadline.

HMRC considers reasonable-excuse appeals on a case-by-case basis. Examples may include:

• A serious illness or unexpected hospital stay
• A bereavement close to the deadline
• Fire, flood or another major incident affecting your records
• A serious computer or software failure
• Problems with HMRC’s online service
• An unexpected postal delay, where you can show you took reasonable care

The important points are that the event must have prevented you from filing or paying on time, and you should act as soon as reasonably possible after the problem has ended.

For example, being unwell for several weeks may be relevant. Continuing to delay for several months after recovering is less likely to support an appeal.

HMRC may expect you to provide:

• The dates when the problem started and ended
• An explanation of how it affected your ability to file or pay
• Evidence, where available
• The date you submitted the return or made payment
• Details of the steps you took to put things right

You generally need to appeal within 30 days of the date on the penalty notice. You can appeal a Self Assessment penalty through GOV.UK.

An accepted reasonable excuse may lead to a penalty being cancelled or reduced. It does not usually remove interest, because interest reflects the period during which HMRC was without the tax payment.

What should you do if you are already late?

Take a practical three-step approach rather than waiting for the situation to resolve itself.

1. Submit the return

Complete and submit the return as soon as you can. You may need to gather:

• Sales and business income records
• Business expenses and receipts
• Bank statements
• Pension contributions
• Dividend and employment information
• Property or investment income
• Details of payments on account already made

If you are a sole trader, make sure your figures are based on proper business records. If you are a company director, keep your personal return separate from the company’s accounts and Corporation Tax responsibilities.

2. Pay what you can

Pay the full amount if possible. If you cannot, make a payment towards the bill and contact HMRC about your options.

Do not assume that waiting until you have enough money is the best approach. Filing the return gives you a clearer figure, helps prevent further filing penalties and allows you to discuss the position with HMRC.

Remember to allow time for payments to reach HMRC. The GOV.UK payment guidance explains the available payment methods and processing times.

3. Review any penalty notice

Check whether the penalty is correct. Look at:

• The filing deadline HMRC has used
• The date your return was received
• The amount of tax outstanding
• Whether a reasonable excuse applies
• The appeal deadline shown on the notice

If the penalty appears wrong, or you had a genuine reasonable excuse, prepare your appeal promptly. Keep copies of your return, payment confirmations, correspondence and supporting evidence.

How can you avoid missing the next deadline?

A simple system can make future Self Assessment deadlines easier to manage.

Consider the following:

• Put the registration, filing and payment dates in your calendar
• Set reminders several weeks before each deadline
• Keep business records updated throughout the year
• Put money aside regularly for tax
• Check whether payments on account will apply
• Avoid leaving records and calculations until January
• Ask for help before the deadline if your circumstances have changed

Cloud bookkeeping software such as Xero can help you keep income and expenses organised, but software does not replace professional review. A qualified accountant can also help you understand how much to reserve for tax and whether your payments on account are appropriate.

At Harrison’s Accountancy, we provide Self Assessment and tax return support for sole traders, company directors and individuals. We can help you organise your records, prepare your return, check figures and plan for future tax bills.

The key message

A missed deadline is a problem to resolve, not a reason to panic.

The costs can increase when a return or payment remains outstanding, so act quickly:

1. Register with HMRC if you need to
2. File the return
3. Pay the tax or agree a payment arrangement
4. Check whether a reasonable-excuse appeal applies
5. Put reminders and record-keeping arrangements in place for next time

If you are unsure what applies to you, contact Harrison’s Accountancy for a confidential discussion. Getting advice early can help you understand the position and choose the most practical next step.