Do employees need to file?

Usually not, if all your income is taxed at source. But you may need to file if you have untaxed income, such as rental income or dividends, or if you're self-employed.

The SA100 is the main Self Assessment tax return form. You fill it in with your personal details and income, and may need supplementary pages for specific income types.

When do I need to register for Self Assessment?

You must register by 5 October after the end of the tax year. For example, for the 2023/24 tax year, register by 5 October 2024.

What is the deadline for filing and paying?

Both filing your return and paying any tax due are due by 31 January following the end of the tax year.

A UTR (Unique Taxpayer Reference) is a 10-digit number HMRC gives you when you register. You'll need it to file your return.

Self Assessment — definition

Self Assessment is HMRC's system for reporting income not taxed at source, like self-employment, property, or dividends. You register by 5 October, get a UTR, and file your return (SA100 plus supplementary pages) by 31 January after the tax year, paying any tax due by the same date.

Reviewed 18 August 2026 · Software guidance, not tax advice.

In one paragraph

Self Assessment is how you tell HMRC about income that isn't taxed automatically. If you're self-employed, have rental income, or receive dividends, you'll likely need to file a return. You register for Self Assessment, get a Unique Taxpayer Reference (UTR), and submit your tax return each year.

Who must file

You must file if you're self-employed, a partner in a business, or have untaxed income like rental income, tips, or investment income. You may also need to file if your income is over a certain amount or you've received certain benefits. If you're an employee with tax deducted at source, you usually don't need to file unless you have other income.

Key dates

5 October – registration deadline for the previous tax year.
31 January – filing deadline and payment deadline for the tax year.

For example, for the 2023/24 tax year, you register by 5 October 2024 and file by 31 January 2025.

Related terms

What is a UTR number?
[Payments on account explained]

Frequently asked questions

Do employees need to file?: Usually not, if all your income is taxed at source. But you may need to file if you have untaxed income, such as rental income or dividends, or if you're self-employed.
What is the SA100?: The SA100 is the main Self Assessment tax return form. You fill it in with your personal details and income, and may need supplementary pages for specific income types.
When do I need to register for Self Assessment?: You must register by 5 October after the end of the tax year. For example, for the 2023/24 tax year, register by 5 October 2024.
What is the deadline for filing and paying?: Both filing your return and paying any tax due are due by 31 January following the end of the tax year.
What is a UTR?: A UTR (Unique Taxpayer Reference) is a 10-digit number HMRC gives you when you register. You'll need it to file your return.