Self assessment and tax returns
Self assessment is a way of reporting your taxable income and paying tax. Under self assessment you submit a tax return to HMRC, giving details of your income and gains for a whole tax year. Not everyone is required to submit an annual tax return – it will depend on whether you meet HMRC’s self assessment criteria.
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Introduction
A UK tax year runs from 6 April to the following 5 April. Therefore, the 2025/26 tax year starts on 6 April 2025 and finishes on 5 April 2026.
Under self assessment, you file a tax return for a tax year which includes all your taxable income for the year – including sources from which you have had tax deducted already. If you have any capital gains, you may need to report these too. You also claim any allowances or reliefs (such as for employment expenses) that you are entitled to on the tax return.
In most cases, HMRC will calculate the tax and other payments you owe based on the information entered on the form. This can include income tax, capital gains tax, class 2 and 4 National Insurance contributions (if you are self-employed) and student loan repayments. If your tax calculation shows you have paid too much tax for the tax year, HMRC will repay you what you have overpaid. See our page on tax refunds for more information on this.
If you file your tax return online using HMRC’s online portal, HMRC will calculate what you owe and you will see the amount you owe at the point you submit the return. If you file your return on paper by the normal paper filing deadline, then HMRC will calculate the tax due and issue a tax calculation in the post once they have processed the form. You should receive this ahead of the 31 January payment deadline so that you can make your payment. You can read more about paying tax under self assessment on our Self assessment tax payments page.
When a tax return is required
Not everyone needs to do a self assessment tax return. Usually, it will depend on whether you fall within HMRC’s self assessment criteria, which typically covers situations such as self employment, rental income and investment income over a certain level. Many people in the UK pay tax through Pay As You Earn and are not required to submit a tax return.
It is your responsibility to tell HMRC if you need to complete a tax return. See our page Who has to complete a tax return for information about HMRC’s self assessment criteria.
You must file a tax return if HMRC ask you to, unless they withdraw that requirement by cancelling the tax return. You can ask them to remove you from self assessment if you no longer meet the criteria. You can read more about this on our page Tax returns: leaving self assessment.
Overview of dates and deadlines
If you need to file a self assessment tax return each year, there are several important dates throughout the year by which you may need to take some action. We give an overview of these dates below. Not all the dates listed below may apply to you.
If the tax year in question is 2025/26, then the 31 January during the tax year is 31 January 2026, the 5 October following the end of the tax year is 5 October 2026 and the 31 January following the end of the tax year is 31 January 2027.
| 31 January (during the tax year) |
If you have to make payments on account, the first payment on account for the tax year ending the following 5 April is due. Not everyone has to pay these payments on account. For example, the first payment on account for the 2025/26 tax year is due by 31 January 2026. |
| April (after the end of the tax year) |
The tax year ends on 5 April and shortly after this date HMRC may send you a notice advising that you must file a tax return for the tax year just ended. |
| 31 July (following the end of the tax year) |
If you have to make payments on account, the second payment on account for the tax year ending the previous 5 April is normally due. For example, the second payment on account for the 2025/26 tax year is due by 31 July 2026. Not everyone has to pay these payments on account. |
| 5 October (following the end of the tax year) |
If you need to file a tax return for a year but HMRC has not asked you to do so, you must notify HMRC by 5 October. Therefore, if you met the self assessment criteria for the first time during the2025/26 tax year, you need to register for self assessment by 5 October 2026. If you miss this registration deadline, you may suffer a failure to notify penalty. |
| 31 October (following the end of the tax year) |
If you are sending HMRC a paper tax return, this must usually be submitted by 31 October. If you send the form after this date there may be a penalty even if you have no tax to pay. You may be able to appeal against the penalty if you have a reasonable excuse for the late filing. |
| 30 December (following the end of the tax year) |
If you file your tax return online, you will need to submit it by this date if you want HMRC to collect the tax through your PAYE tax code. This may be possible where you owe less than £3,000. If your income is more than £30,000, even more tax may be collected through your tax code. |
| 31 January (following the end of the tax year) |
All tax returns filed online must usually be submitted on or before 31 January following the end of the tax year. If you miss this deadline a penalty will be charged, even if you have no tax to pay or have already paid all the tax you owe. You may be able to appeal against the penalty if you have a reasonable excuse for the late filing. Your balancing payment of tax is also due on 31 January following the end of the tax year. For example, your balancing payment for 2025/26 is due on 31 January 2027. You may also have a payment on account to make at this time. For example, you may have a payment on account to pay for the 2026/27 tax year on 31 January 2027. Not everyone has to make payments on account. |
| 31 January (following the end of the tax year) + 1 year |
If you become aware that an entry on your paper or online tax return is incorrect you can amend that return up to 12 months after 31 January following the end of the tax year. This deadline applies even if the return was filed late. However, it is extended in the case where the notice to file a return is given after 31 October following the end of the tax year. For example, if you need to amend your 2024/25 return you have until 31 January 2027 to make the amendment. This applies whether you filed using a paper return or completed it online. |
Methods of filing a tax return
A self assessment tax return, also called an SA100, can either be filed electronically (online) or on paper. Usually, you can choose whether you file online or on paper.
Online tax return
HMRC prefer as many people as possible to file their tax return online. If you file your tax return online, you usually have longer to do it. You can file online using HMRC’s online self assessment service, which can be accessed via your Personal Tax Account on GOV.UK, or using third-party commercial software.
However, certain taxpayers cannot use HMRC’s online self assessment service because of software limitations. This includes anyone who needs to file the SA109 Residence, remittance basis, etc. supplementary pages, or if you had taxable trust or estate income or are in a partnership. If this applies to you, you will need to either use third-party commercial software, file on paper, or get an agent to file your tax return for you.
In limited circumstances, some taxpayers cannot file their tax return electronically at all and therefore must file a paper return.
Paper tax return
If you need to file your tax return on paper, HMRC might send you the form automatically in the post after the end of the tax year.
If you do not receive a paper return but need or prefer to file on paper, the blank SA100 tax return can be downloaded from GOV.UK, or you can call HMRC to request that a copy is sent to you in the post.
Short tax return
HMRC may also issue a short tax return, on paper, in some cases. You cannot download the short tax return. You can only use the short tax return if HMRC ask you to. If you are not issued with an actual tax return but instead you are sent a notice requiring you to complete a tax return, you can request that a short tax return be issued to you, if appropriate. Filing dates are the same as for the full self assessment return.
Getting help
If you get a tax return, there is information on our website which may help you. You can also contact HMRC for help. If you are concerned about completing the form, you can contact HMRC.
Alternatively, see our Getting help page for details of the tax charities who may be able to offer free assistance.
