How to Do Your Self Assessment Tax Return in the UK (2025–26 Guide)
- Who needs to file a Self Assessment tax return?
- Key dates and deadlines to know
- How to register for Self Assessment
- What information you need to complete your return
- Allowable business expenses that reduce your tax bill
- Payments on Account: the trap that catches new filers
- How to actually file: online step by step
You must file a Self Assessment tax return if you are self-employed, a limited company director, or earn over £100,000. The deadline for online filing is 31 January each year (for the previous tax year ending 5 April). Register with HMRC first if it is your first return — do this by 5 October in the tax year after the one you need to report.
- Who needs to file a Self Assessment tax return?
- Key dates and deadlines to know
- How to register for Self Assessment
- What information you need to complete your return
- Allowable business expenses that reduce your tax bill
Who needs to file a Self Assessment tax return?#
You need to file a Self Assessment return if any of these apply: you are self-employed as a sole trader; you are a director of a limited company; your income from employment or pensions was over £100,000; you earned more than £1,000 from self-employment, freelancing, or a side hustle; you received income from renting property; you had income from savings and investments above the tax-free allowances; or HMRC has contacted you and told you to file. If you are new to Self Assessment, you must register with HMRC by 5 October in the second tax year of self-employment. Miss this and you face a £100 fine even before you have filed.
Key dates and deadlines to know#
The UK tax year runs from 6 April to 5 April. For the 2025–26 tax year (6 April 2025 to 5 April 2026): the deadline to register for Self Assessment if it is your first year is 5 October 2026; the deadline to file a paper return is 31 October 2026; the deadline to file online and pay any tax owed is 31 January 2027; and a second payment on account for the 2025–26 year is due 31 July 2026. Miss the 31 January deadline and you receive an automatic £100 fine, with daily penalties of £10 after 3 months and percentage-based surcharges after 6 months.
How to register for Self Assessment#
If this is your first time, go to www.gov.uk/log-in-file-self-assessment-tax-return and select "Register if you are not self-employed" or "Register if you are self-employed." HMRC will send you a Unique Taxpayer Reference (UTR) number by post — this takes up to 10 working days. You also need a Government Gateway account to file online. Register for that at www.gov.uk/government-gateway. Keep your UTR safe — you will need it every year.
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What information you need to complete your return#
Gather these before you start: all income sources (employment income from P60s, self-employment income, rental income, dividend income, bank interest); business expenses receipts and records; any pension contributions; Gift Aid donations (you can claim higher-rate tax relief on these); your P11D if your employer provided benefits; student loan information; and last year's Self Assessment return as a reference. If you are self-employed, you need your profit and loss figures — income minus allowable business expenses. Good record-keeping throughout the year makes this a 2-hour task rather than a week-long nightmare.
Allowable business expenses that reduce your tax bill#
Business expenses reduce your taxable profit and therefore your tax bill. For self-employed people and company directors, allowable expenses include: office costs (stationery, postage, software), travel costs (not normal commuting, but client travel, mileage at 45p/mile for cars), equipment and tools used for work, professional fees (accountant, solicitor, professional memberships), advertising and marketing costs, business insurance, phone and internet (the business proportion), and training directly related to your current work. You cannot claim for personal expenses, entertaining clients, fines, or capital purchases (which are claimed separately through capital allowances).
Payments on Account: the trap that catches new filers#
If your Self Assessment tax bill exceeds £1,000, HMRC requires you to make Payments on Account — advance payments toward the following year's tax bill. These are due in two instalments: 50% by 31 January (at the same time as settling the previous year's bill) and 50% by 31 July. In your first year this can mean paying 150% of your tax bill by 31 January: the actual tax for the year just ended, plus the first payment on account for the year ahead. Many first-time filers are blindsided by this. If you expect your next year's income to be lower, you can apply to reduce payments on account.
How to actually file: online step by step#
Log in to your HMRC online account at www.gov.uk/personal-tax-account. Select "Self Assessment" and then "Complete your tax return." Work through each section: personal details, income (employment, self-employment, property, investments), reliefs and deductions, and review. The system calculates your tax automatically. Check the figures against your own calculation before submitting. Once submitted, pay any tax owed by 31 January — you can pay by bank transfer, debit card, or set up a Direct Debit. Save a screenshot or the reference number as proof of payment.
People also ask
What is the Self Assessment deadline for 2025–26?
The online filing deadline for the 2025–26 tax return is 31 January 2027. The paper filing deadline is 31 October 2026. Any tax owed must also be paid by 31 January 2027.
How much do you have to earn before filing a Self Assessment?
You must file if you are self-employed and earned more than £1,000 from self-employment, or if you are a company director, or if your income from all sources exceeded £100,000. HMRC may also require you to file for other reasons (rental income, high income child benefit charge, etc.).
Can I do my own Self Assessment without an accountant?
Yes. The HMRC online system is straightforward for simple cases. If you are a sole trader with simple accounts, doing your own return is reasonable. If you have complex affairs (multiple income sources, rental property, foreign income, capital gains), an accountant's fee will typically be covered by the tax they save you.
What happens if I miss the Self Assessment deadline?
Missing 31 January triggers an automatic £100 penalty, even if no tax is owed. After 3 months, daily penalties of £10 apply (up to £900). After 6 months, a further 5% of unpaid tax is added. After 12 months, another 5%. Interest also accrues on unpaid tax from 1 February.
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