TL;DR
Self Assessment is the system HMRC uses to collect tax on income not taxed at source. For the 2025/26 tax year: register by 5 October 2026, file paper return by 31 October 2026, file online by 31 January 2027, pay any tax by 31 January 2027. The late filing penalty starts at 100 pounds and rises sharply after three months.
Last reviewed: 23 June 2026
Key Facts: Self Assessment 2025/26
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What is Self Assessment?
Self Assessment is the system used by HM Revenue and Customs to collect Income Tax and National Insurance from people whose tax cannot be fully handled through Pay As You Earn. Unlike employees whose tax is deducted before pay reaches a bank account, Self Assessment requires the individual to calculate and report their own tax liability each year.
Self Assessment returns are backward-looking. The return filed by January 2027 covers income earned in the 2025/26 tax year, which ran from 6 April 2025 to 5 April 2026.
Who Must File a Self Assessment Tax Return
| Situation | Why Self Assessment Applies |
|---|---|
| Self-employed sole trader earning over 1,000 pounds | Income not taxed at source |
| Partner in a business partnership | Partnership income must be declared |
| Director of a limited company | Additional income outside PAYE |
| Earner of untaxed income over 1,000 pounds | Rental, foreign, or investment income |
| Income over 100,000 pounds | Personal allowance taper must be calculated |
| High Income Child Benefit Charge applies | Income over 60,000 pounds with Child Benefit |
| Capital gains to declare | Gains from property or shares above annual exempt amount |
| HMRC has issued a notice to file | Statutory requirement once notice received |
Source: HMRC - Self Assessment tax returns: who must send a tax return (GOV.UK).
Self Assessment deadlines for 2025/26
The 2025/26 tax year ended on 5 April 2026. The key dates are:
- 5 October 2026: Deadline to register with HMRC if filing a Self Assessment return for the first time for 2025/26. Registration provides a Unique Taxpayer Reference (UTR).
- 31 October 2026: Deadline for paper tax returns. HMRC must receive the physical return by this date. After this date, paper returns are treated as late.
- 31 January 2027: Deadline for online tax returns and for payment of all tax owed for 2025/26. This is also the deadline for first payments on account for 2026/27 where the 2025/26 bill exceeds 1,000 pounds.
- 31 July 2027: Deadline for the second payment on account for 2026/27.
Self Assessment Late Filing Penalties
| How Late | Penalty |
|---|---|
| Up to 3 months late | 100 pounds automatic penalty |
| 3 to 6 months late | 100 pounds plus 10 pounds per day (maximum 900 pounds) |
| 6 to 12 months late | Additional 5% of tax due or 300 pounds, whichever is greater |
| Over 12 months late | Further 5% of tax due or 300 pounds, whichever is greater |
| Late payment (30 days+) | 5% of unpaid tax |
| Late payment (6 months+) | Further 5% of unpaid tax |
| Late payment (12 months+) | Further 5% of unpaid tax |
Penalties are separate from interest charged on unpaid tax. Source: HMRC.
What Making Tax Digital means for Self Assessment filers
From 6 April 2026, Making Tax Digital for Income Tax (MTD for ITSA) is mandatory for sole traders and landlords with combined income from self-employment and property above 50,000 pounds per year. These filers must keep digital records using MTD-compatible software and submit quarterly updates to HMRC, in addition to a final annual declaration.
The threshold reduces to 30,000 pounds from April 2027 and 20,000 pounds from April 2028, meaning most self-employed filers will eventually be within scope.
What to include in a Self Assessment return
The SA100 is the main Self Assessment form. It covers income from employment, self-employment, property, savings, investments, pensions and overseas sources. Supporting pages are required for each income type: SA103 for self-employment, SA105 for UK property income, SA106 for foreign income, and SA108 for capital gains.
Do I need to file a Self Assessment tax return?
A Self Assessment return must be filed if any of the following apply: self-employment income over 1,000 pounds, untaxed income (rental, dividends, foreign), income over 100,000 pounds, a Child Benefit charge where income exceeds 60,000 pounds, capital gains above the annual exempt amount, or if HMRC has issued a notice to file.
What is the Self Assessment deadline?
For the 2025/26 tax year: online return deadline is 31 January 2027, paper return deadline is 31 October 2026. Tax owed must also be paid by 31 January 2027.
What is a UTR number?
A Unique Taxpayer Reference (UTR) is a 10-digit code issued by HMRC when someone registers for Self Assessment. It identifies the taxpayer on all HMRC systems. You receive one UTR for life. New filers must register by 5 October 2026 to receive their UTR in time to file the return.
What is a payment on account?
Payments on account are advance payments toward the next year tax bill. If the Self Assessment bill is over 1,000 pounds, HMRC requires two advance payments - one on 31 January and one on 31 July - each equal to half the previous year tax bill.
What is Making Tax Digital for Income Tax?
Making Tax Digital for Income Tax (MTD for ITSA) requires eligible self-employed individuals and landlords to keep digital records and submit quarterly updates to HMRC using compatible software. It applies from April 2026 for those with income over 50,000 pounds, and will roll out to lower income thresholds in subsequent years.
Can I file a Self Assessment return early?
Yes. The 2025/26 return can be filed any time after 6 April 2026 up to the January 2027 deadline. Filing early means the tax liability is known sooner, giving more time to arrange payment. Figures can be amended after filing if needed.
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