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With the 31st of January deadline fast approaching, HM Revenue and Customs (HMRC) has warned that millions of taxpayers have still not completed their Self-Assessment tax returns online.

But what can you do if you miss the deadline? Mia Odell, at ARAG Law, tells you what you need to know…

You don’t need to wait until January the 31st to file your Self-Assessment Tax return. HMRC is encouraging customers to complete their Self-Assessment tax return as soon as possible, so they know what they owe and can make payment by the 31st of January 2026. This also means if a repayment is due, it can be claimed sooner.

Who should complete a tax return?

You must send a tax return if, in the last tax year any of the following applied:

  • You were self employed as a sole trader and earned more than £1,000 (before taking off anything you can claim tax relief on)

  • You were a partner in a business partnership.

  • You had to pay Capital Gains Tax when you sold of ‘disposed of’ something that increased in value

  • You had to pay the high-income child benefit charge and did not pay it through PAYE.

Or you may need to send a tax return if you have any untaxed income such as:

  • Money from renting a property (rental income)

  • Tips and commission

  • Income from savings investments and dividends

  • Foreign income.

You can check if you need to send a tax return by clicking here.

Other reasons for sending a return – you can choose to fill in a tax return to:

  • Pay voluntary National Insurance contributions

  • Claim some Income Tax reliefs

  • Prove you’re self-employed, for example to claim Tax-Free Childcare or Maternity Allowance

Registering for Self-Assessment

If one of the above categories apply to you, you may need to register for self-assessment by 5th October if:

  • You have not sent a tax return before

  • You have registered before but did not need to send a tax return for the tax year 2023 to 2024

  • To register for Self Assessment please visit www.gov.uk/register-for-self-assessment. You may also need to reactivate your account. Please also be aware that your tax return may be delayed if you register again instead of reactivating an existing account

    Please also be aware that your tax return may be delayed if you register again instead of reactivating an existing account.

  • If you register after 5th October HMRC will send you a letter or email with a different deadline to send your tax return by – this will be 3 months from the date on the letter or email.

    It is important to still pay the tax you owe by 11:59pm on 31st January 2026 or you will recieve a penalty.

How to prepare your tax bill

You will need to pay your tax bill by 31st January. Once you have registered you can prepare to pay by doing the following:

  • Calculate an estimation of how much money you will need to pay

  • Set up monthly or weekly payments to help budget your bill

  • Send your tax return any time after 5 April, doing this earlier means you’ll know what you owe

Sending your return

You can send your tax return by either submitting an online return or sending a paper return. You can send this anytime after 5th April, benefits of filing earlier include: establishing what you owe, setting up a payment plan to better budget paying your tax and you will have time to pay your final bill by 31st January.

  • An online return can be paid using this link – www.gov.uk/log-in-file-self-assessment-tax-return

  • A paper tax return can alternatively be sent by using the SA100 form.

What happens if you don’t know your profit for the whole tax year

There may be situations whereby you do not know what your profit will be for the whole tax year, if for example:

  • You ‘accounting period’ ends at a different time to the end of the tax year

  • Your ‘accounting period’ is different to your ‘basis period’

  • You are awaiting a valuation

What is an accounting period – it is the period which a business’s accounts are made up to (often 12 months) and gives the business year end. This can be different to the period used to identify the profits taxable in any particular tax year which is known as the ‘basis period’.

If you do not know your profit you should calculate provisional figures and include those and let HMRC the figures used are provisional. When you find out what your profit was for the whole tax year you will to change your return within 12 months. In cases where more tax is due you will need to pay interest on the difference, in cases where you have overpaid tax the accrued interest will be paid to you.

You will need to notify HMRC if you no longer need to file a Self-Assessment

When you no longer need to send a tax return:

• If you’ve stopped being self-employed
• If you no longer rent out a property.
• If you no longer pay the High Income Child Benefit Charge.

What to do:

• Sign in to your HMRC account and complete the online form to:
• Close your account, or
• Request removal from Self-Assessment for a specific tax year.

Alternative for Child Benefit Charge:

• You can choose to pay the High Income Child Benefit Charge via PAYE instead of Self-Assessment: Click here

Important note

If you’ve told HMRC you’re no longer self-employed, you may still need to submit tax returns until HMRC confirms removal.

Deadlines

  • Paper return – HMRC must receive this by 11:50pm on 31st October 2025 or you will get a late filing penalty

  • Online return – must be submitted by 11:59pm on 31st January 2026 or you will receive a late filing penalty

  • If you want to pay your Self Assessment bill through your tax code, you must submit it by 11:59 on 30th December, if you miss this you will have to pay another way

Paying the tax you owe

  • You must pay by 11:59 31st January 2026 or you will receive a penalty

  • There is a second payment deadline of 31st July if you make payments towards your bill – ‘payments on account’

Penalties

There are penalties for failing to file your tax return on time or paying the bill late, there is an initially a penalty of £100 if your tax return is up to 3 months late. After 3 months there are additional penalties of £10 per day up to a maximum of £900. Then after 6 months there are further penalties of 5% of the tax due or £300 whichever is greater and after 12 months, another 5% charge.

Paying your tax late will result in penalties of 5% of the unpaid tax at: 30 days, 6 months and 12 months.

But what if you have good reason to file your return late - e.g. your business is recovering from a fire or you have suffered flooding?

You can appeal against a penalty if you have a reasonable excuse within 30 day of the penalty being issued.

HMRC says that a reasonable excuse for missing the deadline is “…normally something unexpected or outside your control that stopped you from meeting a tax obligation.”

Examples include:

• Your partner or another close relative died shortly before the tax return or payment deadline
• You had an unexpected stay in hospital that prevented you from dealing with your tax affairs
• You had a serious or life-threatening illness
• Your computer or software failed while you were preparing your online return
• Issues with HM Revenue and Customs (HMRC) online services
• A fire, flood or theft prevented you from completing your tax return
• Postal delays that you could not have predicted
• Delays related to a disability or mental illness you have
• You were unaware of or misunderstood your legal obligation
• You relied on someone else to send your return, and they did not

Excuses that HMRC will not accept include:

• Your cheque bounced due to not having enough money
• You found HMRC’s online system too difficult to use
• You didn’t get a reminder from HMRC
• You made a mistake on your tax return

If you have a disability and claim to have a reasonable excuse that prevented you from meeting a deadline

Cancelling or appealing a penalty

HMRC may cancel a penalty for late filing in cases where the taxpayer can show that there was a reasonable excuse for failing to file on time. However, that excuse needs to have prevented the taxpayer from filing a return over the whole period – in other words, it must have applied continuously. For example, your case will be considerably weakened if you have worked and received taxable income during the period of the delay. HMRC might well argue that, if you were well enough to work, you were well enough to complete your tax return.

If you are still waiting for information to complete a return, it is entirely legitimate to make a reasonable estimate of the income or gain and then amend the return when the information becomes available. There is no penalty for amending a return, though there is a time limit for doing so.

Taxpayers have the right of appeal in respect of penalties charged and can argue the case in front of a tax tribunal.

Disclaimer: This information is for general guidance regarding rights and responsibilities and is not formal legal advice as no lawyer-client relationship has been created.