Late Self Assessment Penalty UK: What It Costs and How to Appeal

The late Self Assessment penalty in the UK starts at £100 for a return filed even one day after the deadline. After 3 months HMRC adds £10 a day for up to 90 days, then 5% of the tax due or £300 at 6 months and again at 12 months. Paying late brings a separate set of 5% penalties and interest, and all of them can be appealed if you had a reasonable excuse.

HMRC's penalties are mechanical. They are raised by the system on fixed dates, they do not depend on whether you meant to file late, and several of them apply even when no tax is owed. That makes them expensive to ignore and, in the right circumstances, straightforward to challenge. This guide sets out the full scale, shows what a year of delay costs, and explains how to appeal. If you are not sure whether you need to file at all, start with do I need to file a Self Assessment return.

How much is the late Self Assessment penalty in the UK?

The late Self Assessment penalty is a sequence of charges, each triggered by the return still being outstanding on a set date. GOV.UK lists four stages for late filing.

How late the return is	Late filing penalty
1 day	An initial £100 penalty
3 months	Additional daily penalties of £10 a day, up to a maximum of £900
6 months	A further penalty of 5% of the tax due or £300, whichever is greater
12 months	Another 5% of the tax due or £300, whichever is greater

Each stage is added to the ones before it. The £100 is not replaced by the daily penalties; you owe both. A return for the 2025 to 2026 tax year that was due online by 31 January 2027 would attract £100 on 1 February 2027, start to accrue £10 a day from 1 May 2027, and reach the 6-month penalty on 1 August 2027.

The first two stages are fixed amounts. The £100 and the daily penalties are charged whether your tax bill is £50,000, nil or a repayment. Many people are caught by this: an employee whose tax is all collected through PAYE, but who was sent a return because of a one-off item, still gets the £100 if they do not file it.

What does a return that is a year late cost?

A Self Assessment return that is more than 12 months late costs at least £1,600 in late filing penalties alone. The minimum is the sum of the fixed amounts in the GOV.UK scale.

Stage	Minimum penalty	Running total
Initial penalty	£100	£100
Daily penalties, 90 days at £10	£900	£1,000
6-month penalty	£300	£1,300
12-month penalty	£300	£1,600

Where tax is due, the 6-month and 12-month penalties are 5% of that tax if that is more than £300. On a tax bill of £20,000, each of those two penalties is £1,000 rather than £300, and the late filing total becomes £3,000.

HMRC also has an online tool to estimate your penalty for late Self Assessment returns and payments, linked from the GOV.UK penalties page.

What is the penalty for paying Self Assessment tax late?

Late payment has its own scale, separate from late filing. GOV.UK says HMRC charges 5% of the tax unpaid at each of three points.

Tax still unpaid after	Late payment penalty
30 days	5% of the tax unpaid at that date
6 months	A further 5% of the tax unpaid at that date
12 months	A further 5% of the tax unpaid at that date

Interest runs as well, from the day after the due date until the tax is paid. HMRC's interest rates page gives the current late payment interest rate as 7.75% from 9 January 2026. The rate is set at the Bank of England base rate plus 4% from 6 April 2025, so it moves when the base rate moves. Interest is also charged on penalties that are paid late.

The two scales run side by side. Someone who neither files nor pays collects the late filing penalties and the late payment penalties together, with interest on the tax throughout. Filing the return on time, even if you cannot pay, removes the first scale completely. If the problem is cash, HMRC's Time to Pay arrangements let you spread the bill, and our guide to HMRC payments on account explains why the January bill is often larger than people expect.

Illustrative example: Priya owes £8,000 for the 2025 to 2026 tax year. Her online return and her payment are both due by 31 January 2027. She files on 20 August 2027 and pays in full the same day. Her late filing penalties are £100, plus £900 in daily penalties, plus a 6-month penalty of £400, which is 5% of £8,000 and more than the £300 minimum: £1,400 in total. Her late payment penalties are 5% of £8,000 at 30 days and a further 5% at 6 months: £800 in total. She owes £2,200 in penalties on an £8,000 bill, before interest for the period the tax was outstanding. Had she filed on time and paid late, the penalties would have been £800.

Which deadline are you measured against?

Penalties run from the filing deadline that applies to you, and that is not always 31 January. GOV.UK gives these deadlines for the 2025 to 2026 tax year, which ran from 6 April 2025 to 5 April 2026:

• 5 October 2026 to tell HMRC you need to file, if you have not filed before.
• 31 October 2026 for a paper return.
• 30 December 2026 to file online if you want HMRC to collect the tax through your tax code.
• 31 January 2027 for an online return and for paying the tax you owe.

There is one important variation. If you registered after 5 October 2026, HMRC sends a letter or email giving you 3 months from the date on it to file. Your filing deadline moves, but GOV.UK is clear that the payment deadline stays at 31 January 2027. People who register late and wait for the notice can find they have no late filing penalty and a late payment penalty all the same.

A paper return sent after 31 October is late even if it arrives well before 31 January. If you have missed the paper deadline, filing online by 31 January avoids the penalty. Late notification, where you never told HMRC you needed to file, carries a different penalty based on the tax still unpaid. We cover registration in registering for Self Assessment by 5 October.

People who are required to use Making Tax Digital for Income Tax are moving to a different, points-based penalty system for late submissions, with its own late payment penalties. HMRC publishes the rules for that regime separately. The scale in this article is the one that applies to a standard Self Assessment return.

How to appeal a late Self Assessment penalty

You can appeal if you had a reasonable excuse for filing or paying late. GOV.UK says you usually have 30 days from the date the penalty was issued to appeal, and that if you miss that deadline you will need to give a reason for the delay.

1. File the return first. Daily penalties keep running while the return is outstanding. An appeal does not stop them, and HMRC expects the failure to have been put right without unreasonable delay once the excuse ended.
2. Gather the dates. HMRC's appeal guidance says you need the date the penalty was issued, the date you filed the return, the date you paid the tax, and the details of your reasonable excuse.
3. Use the GOV.UK tool. It directs you either to appeal online or to download form SA370, or SA371 for a partnership, and appeal by post.
4. Explain what happened and when. Give the event, its dates, how it prevented you from filing or paying, and what you did as soon as it was over. Attach evidence where you have it.
5. Consider paying the penalty while you wait. HMRC's guidance says that if you do not pay and the appeal is rejected, you will owe interest on the penalty from the date it was due. If the appeal succeeds, HMRC repays what you paid, with interest.
6. Ask for a review if HMRC says no. You can ask HMRC to review the decision, and you can take the appeal to the tax tribunal.

What HMRC accepts as a reasonable excuse

GOV.UK's list of reasonable excuses includes:

• your partner or another close relative died shortly before the deadline;
• you had an unexpected stay in hospital, or a serious or life-threatening illness;
• your computer or software failed while you were preparing your online return;
• there were issues with HMRC's online services;
• a fire, flood or theft prevented you from completing the return;
• postal delays that you could not have predicted;
• delays related to a disability or mental illness;
• you were unaware of, or misunderstood, your legal obligation;
• you relied on someone else to send your return, and they did not.

The same page lists what does not count: a payment that failed because you did not have enough money, finding the HMRC online system too difficult, not getting a reminder from HMRC, and making a mistake on the return.

When no return was needed

A different route applies where you should never have been asked for a return. HMRC's guidance says you can ask for a penalty to be cancelled if you do not need to send a Self Assessment tax return. If HMRC agrees to withdraw the return, the late filing penalties fall away with it. This is worth checking before drafting an appeal, because it does not depend on having an excuse.

Late Self Assessment penalties for Americans and other new arrivals

People who have recently moved to the UK are over-represented among those receiving a first penalty notice, and the reasons are structural.

• The UK tax year is not the calendar year. It runs from 6 April to 5 April, so the return, the registration deadline and the payment date all fall at different points from the US cycle. See UK tax year vs US tax year.
• PAYE hides the obligation. An employee whose salary is taxed at source may still need a return for foreign dividends, US rental income, or gains on US investments, none of which an employer reports.
• You cannot file without a UTR. A Unique Taxpayer Reference has to be issued before an online return can be submitted, and waiting for it close to the deadline is a common cause of late filing. Our guide to the UTR number for Americans in the UK covers the timing.
• The US filing does not count. Filing Form 1040 with the IRS does nothing for the UK obligation, and the US extension dates have no effect on HMRC's deadlines.

GOV.UK's list of reasonable excuses includes being unaware of or misunderstanding your legal obligation. Whether that applies depends on the facts and on how quickly you acted once you found out, so it should be set out carefully and honestly in the appeal, with dates.

What people get wrong about late Self Assessment penalties

• Thinking no tax means no penalty. The £100 and the daily penalties are due regardless of the tax bill.
• Waiting to file until they can pay. Filing on time and paying late is always cheaper than doing both late.
• Ignoring a return they believe is unnecessary. The penalties accrue until the return is filed or HMRC withdraws it.
• Missing the 30-day appeal window. A late appeal needs a reason for its own lateness as well as for the original failure.
• Assuming the appeal pauses interest. If the appeal fails, interest on the penalty runs from its original due date.

Getting a late return filed and the penalty reviewed

If a penalty notice has arrived, the order of work is fixed: file the outstanding return, pay or arrange to pay the tax, then deal with the penalty. Each day of delay in the first step can add to the bill, and a prompt filing strengthens any appeal that follows.

US/UK Cross Border Tax is a team of US CPAs and UK tax advisers working as one team, with offices in London, Manchester, New York and San Francisco. We bring late UK returns up to date, check whether a return was needed at all, and prepare penalty appeals where there is a reasonable excuse, alongside the US filing where one is required. Our UK Self Assessment service covers late and current-year returns, and Americans in the UK can read more about how we work on our Americans in the UK page. To get a late return moving, contact us.