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If you’re self-employed, receive rental income, or have other sources of income not taxed at source, HM Revenue and Customs (HMRC) requires you to complete a Self-Assessment tax return. HMRC uses Self Assessments to collect income tax in the UK. This return is a crucial annual process for declaring your total income and capital gains, allowing HMRC to calculate your tax liability.

Let’s look at what a Self Assessment is, who needs to file one, and the key dates you need to know. It’s important to note that this is for general information only and should not be taken as advice. If you’re unsure, seek independent financial advice.

 

What is a Self-Assessment tax return?

A Self-Assessment tax return is a form which can be filled out online or on paper that you submit to HMRC. The Self Assessment declares all your taxable income and capital gains for a given tax year (which runs from 6 April to 5 April the following year).

Unlike employees whose tax is generally deducted automatically from their wages via the Pay As You Earn (PAYE) system, individuals in Self Assessment must actively report their earnings. The return not only calculates your income tax but can also include Capital Gains Tax and National Insurance contributions for the self-employed (Class 2 and Class 4). Once you submit your return, HMRC calculates the final bill you owe, which you must pay by the due date.

 

Who needs to file a Self-Assessment tax return?

Not everyone needs to complete a Self-Assessment tax return. Generally, if you are employed and have no other income to report, HMRC may not require you to file one.

However, you’ll need to send a tax return if in the last tax year:

  • You were self-employed as a sole trader and earned more than £1,000 (before taking off anything for tax allowances or expenses).
  • You were a partner in a business partnership.
  • You received rental income from land or property (unless your gross income from property was under £1,000).
  • You received untaxed income (e.g. from investments, savings, or foreign sources) and the tax is not otherwise collected.
  • You are a company director (unless your only income is your salary taxed under PAYE and you didn’t receive any benefits in kind).
  • Your total taxable income was over £100,000.
  • You had to pay the High Income Child Benefit Charge (because you or your partner’s income was over a certain threshold).
  • You want to claim tax relief on employment expenses of over £2,500.
  • You have Capital Gains Tax to pay (e.g. from selling shares or a second home).

HMRC may also require you to send a tax return if you have any untaxed income, such as:

  • Money from renting out a property
  • Tips and commission
  • Income from savings, investments and dividends
  • Foreign income

You can check the HMRC website for more information on who needs to file a Self-Assessment tax return.

Important: It’s your responsibility to register for Self Assessment if you meet the criteria, even if HMRC hasn’t contacted you.

 

Key deadlines you need to know

Missing a Self Assessment deadline can lead to fines and penalties, so it’s important to stay organised. All deadlines relate to the tax year that ended on 5 April.

5 October Registration deadline: You must tell HMRC you need to file a tax return for the previous tax year by this date.
31 October (midnight) Paper filing deadline: The deadline for submitting a paper tax return for the previous tax year.
31 January (midnight) Online filing deadline: The deadline for submitting your online tax return for the previous tax year.
31 January (midnight) Payment deadline: The deadline for paying the tax you owe for the previous tax year and your first payment on account for the current tax year.
31 July (midnight) Second payment on account: The deadline for paying your second payment on account for the previous tax year’s tax bill.

 

Payments on account explained

If your last Self Assessment tax bill was over £1,000 and less than 80% of your tax was collected at source – for example, through PAYE – you’ll typically have to make two payments on account for the following tax year. Each payment is half of your previous year’s tax bill and is due on 31 January and 31 July.

 

Filing a paper Self Assessment tax return

While the majority of taxpayers now file online, you still have the option to file a paper return using the SA100 form.

  1. Obtain the forms: Find the correct paper forms for the relevant tax year. You can download and print the main SA100 form and any necessary supplementary pages (like the SA103 for self-employment or SA105 for property) directly from the HMRC website, or you can request a copy be sent to you by calling HMRC’s Self Assessment order line.
  2. Calculate and complete: Unlike the online system, you must manually calculate your tax liability and fill in all relevant fields on the SA100 and any supplementary pages. It’s a good idea to check your figures carefully.
  3. Meet the deadline: The deadline for HMRC to receive your paper return is 31 October following the end of the tax year. For example, for the tax year ending 5 April 2026, the paper deadline is 31 October 2026. Missing this deadline will result in an immediate penalty, even if you have no tax to pay.
  4. Submission: Post the completed forms to the address provided on the return. You could use a tracked or recorded delivery service to ensure you have proof that HMRC received it on time.

Note: Your actual tax payment is still due by the 31 January deadline, regardless of when you filed your paper return.

 

Filing your Self Assessment online

The online filing method offers a faster process and a later deadline (31 January). To file online, you first need to register for Self Assessment and get your Unique Taxpayer Reference (UTR) and a Government Gateway ID. Once logged in to your HMRC online account, the system will guide you through a series of questions.

 

Making Tax Digital

The Self Assessment system is set to change with the introduction of Making Tax Digital for Income Tax Self Assessment (MTD for ITSA). This means affected businesses and landlords will have to use HMRC-compatible software to maintain digital records and file their returns.

The changes will start rolling out in 2026, starting in April 2026 for those with annual gross income over £50,000, and from April 2027 for those with income over £30,000. Instead of a single annual tax return, MTD for ITSA asks individuals to submit quarterly updates of their income and expenses to HMRC, followed by a final declaration at the end of the tax year.

 

 

Source: https://www.gov.uk/browse/tax/self-assessment 

Correct as of October 2025.

 

Moneybox does not offer tax advice. If you’re unsure, seek independent financial advice.