UK Tax Return Self-Assessment Guide (Deadlines, Filing, 2026)

Who Must File

You must file a Self-Assessment tax return if any of the following apply: you are self-employed with profits over £1,000 (the trading allowance threshold); you are a company director (even if your company pays you through PAYE); your total income is over £100,000 (which triggers the personal allowance taper); you have savings or investment income over £10,000; you have capital gains to report (even if below the £3,000 allowance, HMRC may require a return if you disposed of assets); you receive rental income from property; you have foreign income; you or your partner receive Child Benefit and either of you earns over £50,000 (the High Income Child Benefit Charge); you are a partner in a business partnership; or you are a trustee.

Even if none of these apply, you might still choose to file a return to claim tax relief on pension contributions, Gift Aid donations, or to report losses. If you are unsure, HMRC will usually write to you if they think you need to register. You can also use the online tool on gov.uk to check whether you need to file.

Registration

If you need to file Self-Assessment, you must register with HMRC by 5 October following the end of the tax year. For example, for the 2025/26 tax year (ending 5 April 2026), you must register by 5 October 2026. Registration is done online at gov.uk. You will receive a Unique Taxpayer Reference (UTR) number — a 10-digit number that identifies you for tax purposes. You will also need to activate your HMRC online account and set up a Government Gateway user ID and password.

If you are self-employed, you register as a sole trader. If you are starting a new business, register as soon as possible — do not wait until October. You can register even if you are not yet earning, which can help you plan. If you fail to register on time, HMRC may charge a penalty. For partnerships, each partner must register separately, and the partnership itself must also register for a UTR. Once registered, you will receive reminders each year when your return is due.

Deadlines

The key Self-Assessment deadlines are: 5 April — end of the tax year; 5 October — register if you have not filed before; 31 October — paper return filing deadline; 31 January — online return filing deadline and payment deadline (for the tax year that ended the previous 5 April); and 31 July — second payment on account deadline (if applicable). Missing the 31 January deadline triggers an immediate £100 penalty — even if you have no tax to pay. After 3 months, daily penalties of £10 per day (up to £900) start. After 6 months, you face an additional 5% of the tax due or £300 (whichever is higher). After 12 months, another 5% or £300 penalty.

Payment must also be made by 31 January. If your tax bill is over £1,000, HMRC will usually ask you to make "payments on account" — advance payments towards your next tax bill. These are due on 31 January and 31 July, each equal to 50% of the previous year's tax bill. After the return is filed, any balancing payment is due by 31 January. Interest is charged on late payments at the Bank of England base rate plus 2.5%.

Filing Step by Step

Log into your HMRC online account and select the relevant tax year. The return has several sections: employment (from your P60 or P45), self-employment (from your business accounts), property income, capital gains, dividends, savings interest, pension contributions, Gift Aid donations, student loan repayments, and the High Income Child Benefit Charge if applicable. You will also need to report any foreign income, trust income, or miscellaneous income.

The online system calculates your tax automatically as you fill in the figures. At the end, you can see the total tax due, including payments on account for the following year. Check everything carefully before submitting — errors can lead to penalties. Once submitted, you will receive an online confirmation. You can amend a submitted return within 12 months of the filing deadline. HMRC may open an enquiry into any return — they usually have 12 months from the filing date to do so.

Record Keeping Requirements

For Self-Assessment, you must keep records to support your return. For business income, you must keep records for at least 5 years after the 31 January filing deadline. For other income (rental, investments), you must keep records for at least 6 years. From 2026, Making Tax Digital (MTD) for Income Tax is being phased in — businesses and landlords with income over £50,000 must use compatible software to keep digital records and submit quarterly updates to HMRC. This will extend to those with income over £30,000 in 2027.

Records to keep include: all receipts and invoices (sales and expenses), bank statements, credit card statements, mileage logs, dividend vouchers, interest certificates from banks, P60s and P45s, details of charitable donations, pension contribution statements, and records of any capital gains or losses. If you keep good records throughout the year, filing your Self-Assessment will be straightforward — and you are less likely to miss deductions or make errors.

Penalties and Corrections

Penalties for late filing escalate quickly: £100 immediately (even if no tax is due), then £10 per day after 3 months (up to £900), then 5% of the tax due (or £300, whichever is higher) after 6 months, and another 5% after 12 months. Late payment penalties are 3.25% (base rate + 2.5%) on overdue tax. Errors in your return can result in penalties of up to 100% of the underpaid tax if HMRC considers the error to be deliberate and concealed. Even careless errors can mean penalties of 30% of the extra tax.

If you make a mistake, you can correct your return within 12 months of the 31 January deadline. Use the "amend a return" facility on your HMRC account. If HMRC has already opened an enquiry, you must tell them directly. If you have a reasonable excuse for missing a deadline (serious illness, bereavement, technical issues, or HMRC delays), you can appeal the penalty. HMRC has a "reasonable excuse" guidance on its website — the bar is set quite high, but genuine cases are typically accepted.

HMRC Enquiries and Investigations

HMRC has the right to open an enquiry into any tax return within 12 months of the filing deadline (or 12 months after the return was actually filed, if later). Enquiries can be random ("aspect enquiries") or targeted at specific risk areas. HMRC uses sophisticated data matching and risk assessment to identify returns that may be incorrect. Common triggers for an enquiry include: large changes in income or expenses compared to previous years; unusually high expenses relative to income; consistently making losses; claims for working from home or travel expenses that seem excessive; property income that does not match expected rental values; and foreign income or assets that are not fully disclosed.

If HMRC opens an enquiry, they will write to you asking specific questions or requesting documents. You have 30 days to respond, though extensions are usually granted if requested. Responses should be full and timely — HMRC can impose penalties for non-compliance. Most enquiries are resolved at the "desk-based" stage (letters and documents) without a face-to-face meeting. If HMRC finds an underpayment, you will be charged the additional tax plus interest and possibly penalties (0-100% depending on whether the error was careless, deliberate, or deliberate and concealed). If HMRC finds no error, the enquiry is closed with no further action. You can ask for a review or appeal to the First-tier Tribunal if you disagree with HMRC's findings. Using a tax agent or accountant helps significantly in dealing with HMRC enquiries — they handle the correspondence and ensure your responses are complete and accurate.

Making Tax Digital for Income Tax

Making Tax Digital (MTD) for Income Tax is being phased in from 2026. If your self-employment or property income exceeds £50,000 per year, you must use MTD-compatible software to keep digital records and submit quarterly updates to HMRC (by 5 August, 5 November, 5 February, and 5 May), followed by an End of Period Statement (EOPS) and final declaration by 31 January. In 2027, the threshold drops to £30,000, and eventually to all self-employed and landlords. MTD replaces the annual Self-Assessment tax return with a more frequent digital reporting system.

MTD-compatible software includes Xero, QuickBooks, FreeAgent, Sage, and many others. These platforms integrate with your bank accounts to automate transaction recording and calculate your tax position in real time. The quarterly updates are cumulative — the fourth quarter update covers the full year. At the end of the year, you submit the final declaration, which is effectively the confirmation of the year's figures. MTD is designed to reduce errors, improve tax compliance, and give businesses a clearer real-time view of their tax liability. Adapting to MTD early is advisable — choose your software, set up digital record keeping, and familiarise yourself with the quarterly reporting cycle.

Using a Tax Accountant or Filing Yourself

Whether to file your own Self-Assessment or use a tax accountant depends on the complexity of your affairs. For straightforward cases — salaried employment with some investment income or a small side business — filing yourself through the HMRC online system is manageable. The system walks you through each section and calculates the tax automatically. HMRC's online help and guidance notes explain each field. For most basic cases, the process takes 2-4 hours in total.

For more complex situations — multiple income sources, foreign income, complex capital gains, trusts, partnerships, or large pension contributions — a qualified accountant or tax advisor is worth the cost (typically £200-£500 for a straightforward return, more for complex cases). An accountant can identify deductions and reliefs you might miss, ensure your return is accurate, and deal with HMRC enquiries if they arise. They can also help with tax planning, not just compliance. The cost of an accountant is tax-deductible as a business expense if you are self-employed. For limited company directors, accountancy fees are usually essential given the additional reporting requirements (Confirmation Statement, Corporation Tax return, Annual Accounts).

FAQs

Do I need to register for Self-Assessment?

You need to register if you are self-employed earning over £1,000, a company director, earn over £100,000, have rental income, capital gains, or foreign income. Use the gov.uk online tool to check.

What is the deadline for filing Self-Assessment?

Online filing: 31 January following the end of the tax year. Paper filing: 31 October. The 2025/26 return (ended 5 April 2026) is due by 31 January 2027.

What happens if I miss the deadline?

An immediate £100 penalty. After 3 months, daily penalties of £10 per day (up to £900). After 6 months, 5% of the tax or £300. Interest also applies on late payments.

How do I pay my Self-Assessment tax?

Pay online through your HMRC account via debit card, credit card, or bank transfer. You can also set up a direct debit or pay through your bank's online service.

What are payments on account?

Advance payments towards your next year's tax bill, each equal to 50% of the previous year's bill. Due on 31 January and 31 July. You can reduce them if your income is falling.