A self assessment tax return is how HMRC collects income tax from people whose tax is not automatically deducted through PAYE — primarily the self-employed, but also others with additional untaxed income. The process can feel daunting if you have never done it before, but it follows a clear, predictable structure each year.
Who needs to file a self assessment tax return?
You generally need to file a self assessment return if you are self-employed and earned more than the trading allowance threshold, you are a partner in a business partnership, you have rental income above a certain threshold, you have significant savings or investment income, your income is over £100,000, you need to pay the High Income Child Benefit Charge, or you have capital gains to report. HMRC also requires returns from company directors in some circumstances. If you are unsure whether you need to file, the gov.uk online checker tool can confirm based on your circumstances.
Key dates and deadlines
The UK tax year runs from 6 April to 5 April. If you need to file a return for the first time, you must register with HMRC by 5 October following the end of the relevant tax year. The deadline for online returns is 31 January following the end of the tax year — for example, the return for the 2025/26 tax year (ending 5 April 2026) would be due by 31 January 2027. Paper returns have an earlier deadline of 31 October.
Payment of any tax owed is also due by 31 January. If your tax bill is above a certain threshold, you may also need to make a "payment on account" — an advance payment towards the following year's tax bill — due in two instalments, on 31 January and 31 July.
How to register for self assessment
If this is your first time filing, you need to register with HMRC, which can be done online at gov.uk. Once registered, HMRC issues you a Unique Taxpayer Reference (UTR) — a 10-digit number that identifies your tax record and is required for all future filings. You will also need to set up a Government Gateway account if you do not already have one, which is used to access your online tax account.
What information do you need?
To complete your return, you will typically need details of your income from all sources, including self-employment profits, employment income (shown on your P60 or P45), rental income, savings and investment income above your Personal Savings Allowance, and any other taxable income. You will also need records of allowable expenses if you are self-employed, details of pension contributions, and information about any reliefs or allowances you wish to claim, such as Marriage Allowance.
Allowable expenses for the self-employed
If you are self-employed, you can deduct legitimate business expenses from your income before calculating your tax liability. Common allowable expenses include office costs, travel costs for business purposes, costs of goods sold, staff costs, and a proportion of home costs if you work from home. Keeping organised records throughout the year — rather than trying to reconstruct them in January — makes the process significantly easier.
How is your tax calculated?
Once you have entered all your income and expenses, HMRC's online system calculates your tax liability automatically, applying your Personal Allowance (see our guide to the Personal Allowance) and the relevant income tax bands (see our guide to income tax bands). It also calculates Class 4 National Insurance if you are self-employed — see our guide to National Insurance for more detail.
Penalties for late filing
Missing the filing deadline results in an automatic £100 penalty, even if you have no tax to pay or have already paid the tax owed. Further penalties accrue the longer the return remains outstanding, plus interest on any unpaid tax. If you have a reasonable excuse for filing late, you can appeal the penalty, but it is always best to file on time or as close to the deadline as possible.
Should you use an accountant?
Many self-employed people manage their own self assessment, particularly if their affairs are straightforward. However, if your situation is more complex — multiple income sources, property income, capital gains, or significant business expenses — a qualified accountant can often save you more in correctly claimed reliefs than their fee costs, as well as reducing the risk of errors.