Self Assessment Late Filing Penalty: What Happens If Your Tax Return Is Late?

Self Assessment late filing penalties at a glance

These penalties are cumulative. Therefore, receiving the initial £100 penalty does not mean the cost stays at £100 if you continue to leave the return outstanding.

What is the Self Assessment deadline for 2025/26?

The 2025/26 tax year ran from 6 April 2025 to 5 April 2026.

For most taxpayers, the main deadlines are:

• 5 October 2026 — tell HMRC if you need to complete a return and need to register or reactivate Self Assessment.
• 31 October 2026 — HMRC must normally receive a paper return.
• 30 December 2026 — earlier online filing deadline if you want eligible tax collected through your PAYE tax code.
• 31 January 2027 — online filing deadline and main Self Assessment payment deadline.

HMRC may give you a different filing date if your notice to file is issued later.

For more detail, see Real Key Accountancy’s Self Assessment tax return deadlines 2026/27 guide.

Do you get a £100 penalty if you do not owe any tax?

Yes. Under the current rules, the initial £100 late filing penalty can apply even if your Self Assessment calculation shows that you owe no tax.

It can also apply if you paid the tax you owed by 31 January but submitted the actual return late. Filing and payment are separate obligations.

That distinction is important. A taxpayer could therefore have no late-payment penalty but still receive a late-filing penalty because the return itself was not submitted on time.

Example: no tax due but the return is late

Example: A sole trader needs to submit a 2025/26 Self Assessment return. After expenses and allowances, there is no additional tax to pay.

They forget to file by 31 January 2027 and submit the return several days later.

Although the tax bill is £0, the return was filed after its deadline. The £100 initial late filing penalty can still apply.

What happens when a Self Assessment return is more than three months late?

Leaving a return outstanding beyond three months can make the penalty considerably larger.

HMRC can charge £10 for each additional day the return remains late, for up to 90 days. The maximum daily penalty is therefore £900.

Combined with the original £100 penalty, the standard late filing penalties can already reach £1,000 before the six-month point.

If you have missed the deadline, filing as soon as possible is therefore generally better than waiting until you have received every penalty notice.

What happens after six months?

When the return reaches six months late, a further penalty can apply.

This is the greater of:

• £300, or
• 5% of the tax liability that should have been shown on the return.

This penalty sits on top of the £100 initial penalty and any daily penalties already incurred.

Example: a return more than six months late

Example: A freelancer submits a required return more than six months after the filing deadline and has a relatively small tax liability.

Their standard filing penalties could include:

• £100 initial penalty
• up to £900 of daily penalties
• £300 six-month penalty

That could mean £1,300 in late filing penalties, before considering any penalty or interest for paying the actual tax late.

What happens when a tax return is more than 12 months late?

Another penalty applies when a return remains outstanding for 12 months.

For an ordinary non-deliberate failure, this is generally another £300 or 5% of the tax liability, whichever is greater.

However, the position becomes more serious if HMRC determines that information was deliberately withheld by failing to submit the return. Depending on the behaviour involved, the 12-month tax-based penalty can be substantially higher. HMRC’s guidance provides for higher percentages where information was deliberately withheld or deliberately withheld and concealed.

If you have returns that are already more than a year overdue, dealing with them promptly can help prevent the situation becoming more complicated.

Late filing and late payment penalties are different

Missing the Self Assessment filing deadline and failing to pay your tax on time are separate issues.

Under the current regime, tax that remains unpaid can attract late-payment penalties of 5% of the unpaid tax at 30 days, six months and 12 months. HMRC also charges interest on overdue tax.

Therefore, someone who has both an overdue tax return and an unpaid tax bill may face:

• late filing penalties;
• late payment penalties; and
• late-payment interest.

If you cannot pay everything immediately, do not use that as a reason to leave the tax return unfiled. Filing the return establishes what you actually owe and stops further filing penalties from continuing to build.

Can you appeal a Self Assessment late filing penalty?

Yes. You can appeal if you believe the penalty is incorrect or you had a reasonable excuse for missing the deadline.

HMRC says you will usually need to appeal within 30 days of the date the penalty was issued. If you appeal later, you will normally need to explain why the appeal itself was delayed.

HMRC provides further instructions in its Self Assessment penalty appeal guidance.

If you have not yet filed the outstanding return, you should normally deal with it as soon as you can rather than waiting for the appeal process to finish.

What counts as a reasonable excuse?

Whether something amounts to a reasonable excuse depends on the circumstances.

HMRC gives examples of situations that may qualify, including:

• the death of a close relative shortly before the deadline;
• an unexpected hospital stay;
• a serious or life-threatening illness;
• computer or software failure while preparing the return;
• problems with HMRC’s online services;
• fire, flood or theft;
• unforeseen postal delays; and
• circumstances connected with a disability or mental health condition.

A key point is that you should deal with the outstanding return as soon as you are reasonably able once the problem has ended.

Read HMRC’s guidance on reasonable excuses before preparing an appeal.

HMRC also gives examples that it would not normally accept, including simply not receiving a reminder, finding the online system too difficult or making a mistake on the return.

What if HMRC says you need to file but you think you do not?

Do not simply ignore the return.

If HMRC has asked you to submit a Self Assessment return but you believe you no longer need to complete one, tell HMRC as soon as possible. For example, this may be relevant if you have stopped being self-employed or no longer receive the income that originally brought you into Self Assessment.

HMRC can consider removing you from Self Assessment for a particular year. Until that has been agreed, however, you should not assume that the requirement to file has disappeared.

Are Self Assessment penalty rules changing under Making Tax Digital?

Yes. A new points-based penalty system is being introduced alongside Making Tax Digital for Income Tax.

For people required to use Making Tax Digital from 6 April 2026, the new late submission and payment rules apply from the tax year in which they join. However, previous tax years remain under the existing penalty system.

Importantly, HMRC specifically confirms that if you joined Making Tax Digital on 6 April 2026, the current penalties still apply to your 2025/26 tax return due on 31 January 2027.

HMRC has also confirmed there are no penalties for missing a quarterly update deadline during the 2026/27 tax year, although affected taxpayers still have to keep digital records and send the required updates before submitting their tax return. The points-based rules apply more widely as the new regime is phased in.

If Making Tax Digital applies to you, read Real Key Accountancy’s Making Tax Digital for Income Tax 2026 guide and check HMRC’s current MTD penalty guidance.

What should you do if your Self Assessment return is already late?

The most useful first step is usually to deal with the outstanding return rather than allowing penalties to continue accumulating.

A practical approach is to:

1. Confirm which tax return is outstanding. Check the tax year and the filing deadline shown by HMRC.
2. Bring your records together. Gather business income, expenses, bank records, invoices and other relevant information.
3. Complete and submit the return promptly. Further penalties can arise while it remains outstanding.
4. Check the tax calculation. Establish whether you also have unpaid tax.
5. Pay what you can by the relevant deadline. If payment is already overdue, check your options with HMRC.
6. Review any penalty notice. If you believe you have a reasonable excuse, consider whether an appeal is appropriate.
7. Improve the process for the next return. Regular bookkeeping can make future filing much easier.

Keeping income and expenses organised throughout the year is usually much easier than rebuilding a year’s records shortly before a deadline.

How Real Key Accountancy can help

If your bookkeeping is incomplete or you need support preparing your return, Real Key Accountancy provides bookkeeping, Self Assessment and related accounting support for sole traders, self-employed professionals and small businesses.

You can explore Real Key Accountancy’s accountancy and Self Assessment services if you need help getting records organised or preparing an outstanding return.

The sooner incomplete records are reviewed, the more time there is to identify missing transactions, obtain documents and establish the correct figures.

Self Assessment late filing penalty: key points

A Self Assessment tax return does not need to be months overdue before penalties begin. Under the current regime, filing after the deadline can trigger an immediate £100 penalty, with further charges after three, six and 12 months.

For the 2025/26 tax year, most online returns must be filed by 31 January 2027, and the existing late filing penalty regime still applies.

If you have already missed a filing deadline, submit the outstanding return as soon as possible. If there was a genuine reason why you could not file on time, review HMRC’s reasonable-excuse and