Skip to main content What is Self Assessment?
Definition of Self Assessment
Self Assessment is a system that HMRC uses to collect Income Tax. Individuals who have earned income that HMRC doesn't yet know about, such as profit from a business, usually have to report that income to HMRC in a Self Assessment tax return.
The tax return shows how much an individual has earned, and from what sources, over the course of a tax year. Most business owners have to prepare a tax return and then send it to HMRC and pay their tax bill by midnight on 31st January every year.
Check out our dedicated guide if you’re unsure about whether you have to file a Self Assessment tax return.
How Self Assessment works
There are a number of steps that an individual must undertake to successfully file their Self Assessment tax return and to pay any tax and National Insurance that’s due:
1. Registering for Self Assessment
If an individual identifies that they need to file a Self Assessment tax return for a particular tax year, they should register with HMRC. The deadline for registering is 5th October of the calendar year in which that tax year ended. For example, individuals who have to file their first Self Assessment tax return for the 2023/24 tax year, which ended on 5th April 2024, need to register with HMRC by 5th October 2024. Once registered, individuals won’t need to register for Self Assessment with HMRC again.
2. Completing and filing a Self Assessment tax return
Taxpayers can complete and file their Self Assessment tax return online via the HMRC website or by using tax return software such as FreeAgent. Individuals who cannot file online may also be able to download and complete paper forms.
To complete and file their tax return, sole traders and partners will need various pieces of information, including:
- their UTR number
- records of expenses
- records of invoices
- records of other business income
Limited company directors who have to complete and file a tax return will also need various pieces of information, including:
Limited company shareholders may also need to provide proof of any dividends received during the tax year.
FreeAgent’s Self Assessment resource library includes a complete list of all the information a business owner needs to complete and file their tax return.
Self Assessment filing deadlines
The deadline for filing a Self Assessment tax return to HMRC online is midnight on the 31st January that follows the end of the relevant tax year. For example, the deadline for online filing for the 2023/24 tax year (which ended on 5th April 2024) is 31st January 2025.
The deadline for filing a paper tax return to HMRC is the 31st October that follows the end of the relevant tax year. For example, the deadline for paper filing for the 2023/24 tax year is 31st October 2024.
3. Paying any tax and National Insurance owed
Individuals who file their Self Assessment tax return online can view their tax bill in the following places:
- In the ‘View your calculation’ section of the completed return on the HMRC website. This is visible when the individual has completed their return but before they have submitted it.
- In the individual’s personal tax account or business tax account (it can take up to 72 hours after they have submitted their return for this to be available).
If you use FreeAgent you can also view your tax liability in the software. Choose ‘Self Assessment’ from the ‘Taxes’ menu at the top of the screen in FreeAgent and then select the relevant tax year from the list. Choose ‘Tax Breakdown’ from the list of options on the right to see how much you owe.
HMRC will expect individuals to pay any tax and National Insurance that they owe, as well as their first payment on account for the following year, by the payment deadline.
The payment deadline is the same day as the deadline for filing a Self Assessment tax return online: the 31st January that follows the end of the relevant tax year. This means that if you’re filing online, your payment is due on the same day as the tax return itself. Filing your tax return online before the deadline is therefore recommended to avoid missing the payment deadline and incurring a penalty. If you have payments on account to make, the second of these will be due by 31st July.
Individuals who complete a paper tax return will receive their tax bill from HMRC by post after filing.
Information about how to pay a Self Assessment tax bill is available on the HMRC website.
Self Assessment software for business owners
If you're a sole trader, a director of a limited company or an unincorporated landlord, FreeAgent's Self Assessment software auto-populates some of your tax return based on the information you enter throughout the year. For sole traders, FreeAgent completes parts of the Self Employment form, and for landlords, FreeAgent completes parts of the UK Property form. When it's time to file, you can submit your Self Assessment tax return online directly to HMRC from within FreeAgent.
Disclaimer: The content included in this glossary is based on our understanding of tax law at the time of publication. It may be subject to change and may not be applicable to your circumstances, so should not be relied upon. You are responsible for complying with tax law and should seek independent advice if you require further information about the content included in this glossary. If you don't have an accountant, take a look at our directory to find a FreeAgent Practice Partner based in your local area.