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Around 12 million Britons were expected to file a self assessment tax return last year, so it's safe to say a good chunk of the population will need to do the same by 31 January 2026.
You usually need to do a self assessment when you're self-employed or receive other untaxed income, for example from investments, savings interest or renting out a property.
But the process can feel like wading through a quagmire, especially if you're filing your self-assessment for the first time.
We've put together a guide to completing your tax return, running through who needs to file, how to register with His Majesty's Revenue & Customs, how to fill it in, and how to pay your bill.
It's possible to file yourself using HMRC's online portal. For many this should be fairly straightforward, but those with more complex circumstances could find it rather time-consuming.
With this in mind, we also consider whether it's worth seeking help from a professional to complete your return.
> New customers can get their tax return completed for £119 with Taxfix*
Do I need to complete a tax return?
Many people are required to file a tax return – ranging from those who work for themselves full-time, to people who run a side hustle and those who get untaxed income from sources such as renting out a property.
These taxpayers usually need to register for and file a self assessment:
- Self-employed sole traders who earned more than £1,000
- Business partners in a business partnership
- Those who need to pay capital gains tax following the sale of an asset
- Those who need to pay the high income child benefit charge and can't do so through the pay as you earn (PAYE) system
What about other untaxed income?
You might need to file if you have untaxed income from sources such as tips and commission, renting out a property, savings and investments, or foreign income.
When is the tax return deadline?
The deadline to file your tax return online is 31 January each year. Keep in mind you file for the previous tax year, so the deadline for your 2024-25 return is 31 January 2026.
This is also the date by which you must pay your tax bill.
The deadline is earlier if you want to file a paper tax return – 31 October – meaning this has already passed for the 2024-25 tax year.
You should also keep the registration deadline in mind, which is 5 October after the end of the tax year you need to file for.
Payments on account: Don't get caught out...
Payments on account can catch newly self-employed people out.
It's essentially an advance payment towards your estimated future tax bill and is due twice a year, on 31 January and 31 July.
This means if you're completing self assessment for the first time, you'll need to make two payments by 31 January 2026 – one that covers the amount due for the previous tax year, and another that goes towards your next tax bill. Here's an example:
- The amount due for 2024-25 is £5,000, which you pay by 31 January 2026.
- You also must pay £2,500 towards your 2025-26 tax bill by 31 January 2026.
- Another payment of £2,500 is due by 31 July 2026.
If your tax bill for 2025-26 ends up being higher than the total payments you've already made, you'll need to pay the balance by 31 January 2027 plus your next payment on account.
How do I register for self assessment?
You must register for self assessment by 5 October if you need to submit a tax return for the previous tax year's earnings.
You have to register if you haven't sent a tax return before, or are filing again after not needing to for the previous tax year.
What happens if I don't register by the deadline?
HMRC can fine you for missing the 5 October deadline. If you think you might need to file, there's an online HMRC tool you can use to confirm this.
Sometimes HMRC sends letters telling people they need to send a return, called a notice to file. But if you think you need to file, don't wait to receive the notice – HMRC won't always know about your untaxed income.
You can register for self assessment online by creating new sign in details with HMRC and requesting your Government Gateway user ID.
You'll get a unique taxpayer reference (UTR) in the post within 10 working days and a separate letter that has an activation code for your tax account within seven working days.
You should then be able to log in to your business tax account and register for self assessment.
Is the registration process different if I'm not self-employed?
Yes, slightly. If you're not self-employed but still get untaxed income from being a landlord, for example, you need to fill in something called form SA1.
You still need to create an HMRC online account first.
You'll be able to complete form SA1 from your account, after which you should get a UTR number in the post. Then you can add self assessment to your services from your account.
What documents do I need to complete my self assessment?
It's worth gathering the required documents and information before sitting down to do your tax return.
We explain more about the information needed below to help you work out whether it applies to you.
| Documents and information | More details |
|---|---|
| Unique taxpayer reference (UTR) | Your taxpayer number, found in letters from HMRC. |
| National Insurance number | Found in payslips, letters about tax, pensions and benefits. |
| Government Gateway user ID | Log in details for your tax account, received from HMRC. |
| Activation code | Needed to activate your tax account for the first time after registering. |
| Self-employed income information | Your invoices and accounting records, to work out how much you earned. |
| Allowable expenses | Accounting records should show expenses incurred while running your business. |
| Income from dividends | Accounting records and bank statements should show dividends received. |
| Income on savings interest | Banks should give a certificate of interest. For earnings outside of cash Isas. |
| Income you've already paid tax on | If you were employed in the tax year, you need your P60 – also look for a P11D form to show employed benefits. |
| Income from trusts | An R185 form should show this. |
| Rental income | Those who let property need to give information about income earned. |
| Pensions or state benefits income | Letters from the relevant organisations should show this information. |
| Pension or charity contributions | These are needed because you may be eligible for tax relief. |
| Student loan repayments | Check for letters from the student loan provider. |
| Sources: Moneyhelper.co.uk, Freeagent.com, January 2026 | |
How to do a tax return: Filling in the return itself
The good news is that when submitting online, HMRC generally guides you through the process by asking you relevant questions, making sure you only fill in the bits necessary to complete your return.
You'll also be able to see your tax calculation based on the information you enter before you file.
While the online process is fairly straightforward in this sense, it's useful to break down what's required from the individual sections. We've linked to the relevant paper forms, but most people should find it more straightforward to file online – and the paper deadline has already passed for tax year 2024-25.
Form SA100 for all self assessment taxpayers
The main section to complete is form SA100, which deals with:
- Income from pensions, benefits, dividends
- Tax reliefs – pension and charitable contributions, blind person's allowance
- Student loan repayments
- Marriage allowance
- High income child benefit charge
There are then supplementary forms you need to fill in depending on the type of income you earn.
Form SA103 for the self-employed
There are two versions of this section – the one that most self-employed will need to fill in is the shorter version called SA103S. The longer one is only for certain self-employed people in more complex situations.
You need to enter your business turnover and business expenses in this section.
Form SA105 for property income
If you let a property, HMRC asks you to enter the rental income that you receive using form SA105.
One section asks for income you receive from furnished holiday lettings only, while the other asks you for the income you get from other properties.
Form SA108 for capital gains
You need to declare the sale – or disposal – of assets such as property or investments and pay any capital gains tax due using form SA108.
You can enter any allowable costs here, which reduces your total taxable gain. In the example of a property, these could be costs for making improvements to the property, provided these still exist in the asset when it's sold.
When it comes to the sale of investments, allowable costs could any investment platform fees.
> Read more: The best investment platforms: How to choose an account
How to pay self assessment tax
You must pay your tax bill and first payment on account by 31 January.
The quickest way to do this is from your online tax account, choosing 'pay by bank account' which directs you to approve a payment from your online or mobile banking provider.
You can also make payments using your debit card, business credit card, online or telephone banking, Clearing House Automated Payment System (Chaps) or in-branch at your bank or building society.
These should go through the same day or the following day. You can also use payment methods that take a little bit longer – such as Bacs, direct debit or cheque – but you should allow enough time for the payment to reach HMRC by the deadline.
What's the penalty for missing the self assessment deadline?
There are separate fines for both missing the filing deadline and not paying your tax bill on time.
HMRC slaps you with a £100 penalty for filing late. After three months this increases by £10 a day – up to a maximum of £900.
After six months it increases even further, by the greater of 5 per cent of your tax bill or £300. HMRC applies this same charge after 12 months, too.
The fine for paying your tax bill late is 5 per cent, applied at different intervals of 30 days, six months and 12 months. HMRC also charges interest on unpaid tax bills.
