Self Assessment Tax Returns Explained: Who Must File, Deadlines and Penalties

In short: Self Assessment is how HMRC collects Income Tax on income that is not taxed at source. You must send a return if HMRC asks you to, tell HMRC you need to file by 5 October, file online and pay by 31 January, and you may face interest and a penalty if you file or pay late.

Most people in the UK never fill in a tax return, because tax on their wages and pensions is taken off automatically through PAYE. But if you have income that is not taxed at source, from self-employment, rent, savings, investments, or a range of other sources, HMRC needs you to report it, and Self Assessment is the system it uses to do that. Get it right and it is a once-a-year piece of admin; miss the deadlines and the penalties and interest start to build. This article explains, in plain language, what Self Assessment is, who has to file, how to register, the key deadlines, how to pay, and what happens if you are late. The figures and dates here are for the 2025 to 2026 tax year as published by GOV.UK.

What Is Self Assessment?

Self Assessment is the system HM Revenue and Customs uses to collect Income Tax on income that is not taxed automatically.

Self Assessment is the system HM Revenue and Customs (HMRC) uses to collect Income Tax on income that is not taxed automatically. Tax is usually deducted at source from wages and pensions through PAYE, but people and businesses with other income must report it in a Self Assessment tax return. You fill the return in after the end of the tax year it applies to, the tax year ending on 5 April, and you must send a return if HMRC asks you to.

HMRC then calculates what you owe based on what you report. How much tax you pay depends on the Income Tax band you fall into, and there is a separate rate for Capital Gains Tax if you need to pay it, for example because you sold shares or a second home. If you file or pay late, you may have to pay both interest and a penalty.

Who Must Send a Self Assessment Tax Return?

You must send a return if HMRC asks you to, which typically includes people with untaxed income such as self-employment, rental income, or significant savings and investment income.

The common thread is income that PAYE has not already taxed. If tax has not been collected automatically, HMRC needs you to declare that income so the right amount can be worked out.

If you are not sure whether you need to file, GOV.UK has a checker, and it is worth using rather than guessing, because the duty to notify HMRC sits with you. If you receive a notice to file from HMRC, you must complete a return even if you think you owe no tax, until HMRC confirms you no longer need to send one.

How Do You Register for Self Assessment?

You register with HMRC to tell them you need to file, and you must do this by 5 October following the end of the tax year.

You must tell HMRC by 5 October if you need to complete a return for the previous year and you have either not sent a tax return before, or you registered before but did not need to send a return for the 2024 to 2025 tax year. You do this by registering for Self Assessment, and you could be fined if you do not tell HMRC by that date.

Registering matters because it is what generates the credentials and Unique Taxpayer Reference you need to file. If you register after 5 October 2026, HMRC will send you a letter or email giving you a different filing deadline, three months from the date on that letter or email, but you must still pay the tax you owe by 31 January 2027 regardless.

When Are the Self Assessment Deadlines?

Tell HMRC by 5 October, file a paper return by 31 October, and file online and pay by 31 January.

For the 2025 to 2026 tax year, which started on 6 April 2025 and ended on 5 April 2026, the key dates are:

1. Register for Self Assessment by 5 October 2026.
2. Submit a paper return by 11:59pm on 31 October 2026.
3. Submit an online return by 11:59pm on 31 January 2027.
4. Pay the tax you owe by 11:59pm on 31 January 2027.

A couple of extra dates are worth noting. If you want to pay your Self Assessment bill through your tax code, you must file online by 30 December 2026. And there is a second payment deadline of 31 July for those who make “payments on account”, advance instalments towards the following year's bill. If a deadline falls on a weekend or bank holiday, the practical point is simple: aim to be early rather than testing it.

How Do You Fill In and Send Your Return?

You can file online through your HMRC account, or on a paper SA100 form, and you need good records to complete it correctly.

Most people file online, which you can do any time on or after 6 April once the tax year has ended. If you need a paper form, you can download the SA100 or call HMRC and ask for it, remembering the earlier 31 October deadline for paper.

Before you start, keep records such as bank statements and receipts so you can report your income and expenses accurately, this is a GOV.UK requirement, not just good practice. HMRC calculates your bill from what you report, so the quality of your records drives the accuracy of your return. If you get stuck, GOV.UK has guidance on getting help filling in your return.

How Do You Pay, and What If You File or Pay Late?

Pay by 31 January. Filing late starts with a £100 penalty, and paying late brings 5% penalties at 30 days, 6 months, and 12 months, plus interest.

Example: your 2025 to 2026 online return is due on 31 January 2027 but you forget and file on 15 February 2027. You immediately owe the initial £100 late-filing penalty, even if you had no tax to pay. If you still had not filed after three months, daily penalties of £10 a day would start, up to a maximum of £900; after six months a further 5% of the tax due or £300 (whichever is greater) would be added; and after twelve months, another 5% or £300.

Late payment is charged separately. On top of any late-filing penalty, you are charged 5% of the tax unpaid at 30 days, again at 6 months, and again at 12 months, with interest running on the amount owed throughout. You must pay a penalty within 30 days of the date on the penalty notice, and if you have a reasonable excuse you can appeal. The takeaway is that filing and paying are two separate duties, each with its own penalties, so meeting one does not excuse missing the other.

Frequently Asked Questions

Conclusion

Self Assessment is really a sequence of dates: register by 5 October, file by 31 October on paper or 31 January online, and pay by 31 January. The people who avoid penalties are the ones who treat it as an annual rhythm, keeping records through the year, registering in good time, and filing and paying before the deadlines rather than on them. Because late filing and late payment are penalised separately, and interest runs on unpaid tax, the cost of drift adds up quickly. Keep clean records, know your dates, and if your affairs are more complex than a single source of income, get help before the January rush rather than during it.

General information based on GOV.UK guidance “Self Assessment tax returns” (Overview, Deadlines and Penalties) for the 2025 to 2026 tax year, not personal tax advice. Dates and figures change year to year, confirm your own position with a qualified adviser or HMRC before acting.