If you’re wondering what happens if you file your tax return late, the short answer is: HMRC starts charging penalties immediately, and those charges grow the longer the return stays unfiled. There is no grace period. Miss the 31 January online deadline and a £100 penalty is issued automatically, regardless of whether you actually owe any tax.

We see this catch people out every year. Someone has a straightforward return, assumes there’s nothing to worry about, and files a few weeks late without realising the clock was already running. By the time the HMRC letter arrives, the initial fine has sometimes been joined by daily charges as well.

This post sets out the full penalty structure in plain terms, explains how late payment charges sit alongside filing penalties, covers what counts as a reasonable excuse, and gives you a practical steer on what to do if you’ve already missed the deadline.

The moment the Self Assessment deadline passes without a filed return, HMRC issues an automatic £100 late filing penalty. This applies whether you owe £10,000 in tax or nothing at all. The penalty is for missing the deadline, not for having an unpaid tax bill.

For the 2025 to 2026 tax year, the key deadlines are:

• 5 October 2026: deadline to register with HMRC if you are new to Self Assessment
• 31 October 2026: deadline to file a paper tax return
• 31 January 2027: deadline to file your return online and pay any tax owed
• 30 December 2026: earlier deadline if you want to pay through your PAYE tax code

The vast majority of people file online, which means the 31 January 2027 deadline is the one to focus on. Miss it by a single day, and the £100 penalty is raised. There is no mechanism to avoid it after the fact, short of appealing on the grounds of a reasonable excuse (covered later in this post).

One thing worth understanding clearly: the £100 is a filing penalty, not a payment penalty. Those are calculated separately, and both can apply at the same time.

The initial £100 is just the starting point. If the return remains unfiled, HMRC applies further penalties at three, six, and twelve months past the deadline.

Three months late

Once your return is three months overdue, daily penalties of £10 per day begin to accumulate. These run for up to 90 days, meaning a maximum additional charge of £900 on top of the original £100. So if your return is still outstanding three months after the deadline, you could already be facing £1,000 in filing penalties before any tax or interest is calculated.

Six months late

At the six-month mark, HMRC charges a further penalty of either 5% of the tax due or £300, whichever is the greater amount. Even if your tax liability is modest, the £300 floor means the penalty can exceed what you actually owe.

Twelve months late

Another 5% of the tax due or £300 (whichever is greater) is added at twelve months. In the most serious cases, where HMRC believes information was withheld deliberately, the 12-month penalty can rise significantly higher, but that is a separate matter from ordinary late filing.

The cumulative picture is stark. A return filed twelve months late could carry filing penalties of well over £1,600, before any late payment charges or interest are added.

Filing your return on time but paying late still results in penalties. It is worth being clear on this because many people assume that once the return is submitted, their only exposure is the unpaid tax itself.

HMRC’s late payment penalty structure for Self Assessment works as follows:

• 30 days late: a penalty of 5% of the unpaid tax
• 6 months late: a further 5% of the tax still outstanding
• 12 months late: another 5% on top

Interest also accrues on unpaid tax from the deadline date. HMRC’s interest rate is linked to the Bank of England base rate, so the exact figure varies, but it adds meaningfully to the overall cost of a late payment.

This means that someone who files on time but does not pay can end up with a 15% surcharge on their unpaid tax bill across the year, plus interest. And someone who both files and pays late will face the filing penalties and the payment penalties running concurrently.

HMRC does offer a penalty calculator on GOV.UK that gives an estimate of what you might owe in filing and payment penalties combined, though it does not factor in previous credits or outstanding interest from earlier years.

HMRC does allow appeals against late filing penalties where there is a genuine reasonable excuse. The standard is not especially forgiving, but it is real, and it is worth understanding before you assume a penalty is unavoidable.

HMRC accepts the following as potential reasonable excuses:

• The death of a partner or close relative shortly before the deadline
• An unexpected stay in hospital
• A serious or life-threatening illness
• A failure in HMRC’s own online services
• Computer or software failure at the point of submission
• Fire, flood, or theft affecting your records
• Postal delays outside your control
• Delays caused by a disability or mental health condition
• Relying on someone else (such as an accountant) to file, and they failed to do so

What HMRC does not accept as a reasonable excuse includes not receiving a reminder, finding the online system difficult, a lack of funds, or making a mistake on the return itself.

Crucially, if you do have a reasonable excuse, the expectation is that you file and pay as soon as the excuse ends. Sitting on a valid excuse while continuing to delay will not help your position.

If you want to appeal a penalty, you need to do so promptly and in writing. We help clients navigate HMRC appeals when there is a genuine case to make, so if you think you have grounds, it is worth talking it through before you write anything to HMRC.

If the deadline has passed and your return is still outstanding, the most important thing is to file as soon as possible. Every day of additional delay carries a cost, and the difference between filing a week late and filing three months late is significant in penalty terms.

Here is what we would recommend:

1. File immediately. Do not wait until you have everything perfect. File what you can and amend later if needed. Getting the return submitted stops the daily charges from starting at the three-month mark.
2. Pay what you owe, or as much as you can. Partial payment reduces the base on which late payment penalties are calculated. Paying nothing when you could pay something makes the eventual bill larger.
3. Consider whether you have grounds for an appeal. If something genuinely prevented you from filing on time, HMRC’s appeals process exists for exactly that situation. Keep any evidence you have.
4. Check whether you need to be in Self Assessment at all. Occasionally people receive notices to file but are not actually required to submit a return. HMRC can deregister you if this applies, which removes the penalty obligation.

If you have a more complex situation, missed returns from previous years, or are unsure what you actually owe, getting the right support early is far cheaper than letting penalties accumulate further. A Self Assessment tax return service can often sort things out more quickly than people expect.