Self Assessment Tax Return: Who Must File and Key Dates

What Self Assessment actually is

Most employees never touch Self Assessment: employers deduct tax through PAYE. It exists for everyone else, whose income reaches them untaxed. The tax year runs from 6 April to 5 April. Around 12 million people file each year, and the triggers catch a weekend side business, a rented room and a modest dividend payout alike. A notice to file must be obeyed even if you owe nothing.

Who must send a tax return

GOV.UK's must-file triggers for the last tax year:

• Self-employed as a sole trader earning more than £1,000 before claimable reliefs.
• You were a partner in a business partnership.
• You had to pay Capital Gains Tax on something sold or disposed of at a gain.
• You had to pay the High Income Child Benefit Charge, not paid through PAYE.
• You are an off-payroll worker repaying a student or postgraduate loan.

You may also need a return for untaxed income: rent, tips, savings interest, dividends, foreign income, or taxable UK income as a non-UK resident. Running a limited company alone does not force a personal return, but dividends often do. GOV.UK's checker tool gives a yes-or-no answer.

The high-earner question: what about income over £100,000?

Old advice misleads here. A high salary taxed entirely through PAYE does not by itself appear on GOV.UK's current must-file list, so a six-figure employee with no other income may not need a return. What high income changes is your personal allowance: above £100,000 of adjusted net income, the standard £12,570 allowance falls by £1 for every £2 over that line, reaching zero at £125,140, pushing many higher earners into the 40% tax threshold sooner than expected. Practical rule: PAYE only, check the tool; untaxed income on top, or a notice to file, means file.

The deadlines for the 2025/26 return

The 2025/26 tax year ended on 5 April 2026. Four dates matter:

• 5 October 2026 — tell HMRC you need to file, if new to filing or you did not need one for 2024/25. Register later and HMRC sets a different deadline, three months from its letter, but payment stays due 31 January 2027.
• 31 October 2026 — HMRC must receive a paper return by 11:59pm, or a late filing penalty follows.
• 30 December 2026 — file online by this date to have a bill under £3,000 collected through your tax code.
• 31 January 2027 — the online return and the tax you owe are both due by 11:59pm.

Diary all four the day you register.

Payments on account: the January surprise

Payments on account are advance payments towards next year's bill, and they catch first-time filers off guard. Each is usually half of last year's tax, including Class 4 National Insurance if self-employed, due 31 January and 31 July. You must make them unless last year's tax was under £1,000, or over 80% of it was paid outside Self Assessment, for example through your tax code. If income falls, ask HMRC to reduce them online or with form SA303 — but reduce too far and HMRC charges interest on the difference.

What happens if you file or pay late

Late filing and late payment are separate failures, and they stack. Late return: an initial £100 penalty; after 3 months, £10 a day up to £900; after 6 months, a further 5% of the tax due or £300, whichever is greater; after 12 months, another 5% or £300. The £100 applies even if no tax is owed. Late payment: 5% of the tax unpaid at 30 days, 6 months and 12 months, plus interest from the due date. If you cannot pay in full, do not wait: HMRC allows payment plans, and part-payment reduces interest and penalties.

Worked example: a self-employed profit of £40,000

A freelance designer with £40,000 of trading profit in 2025/26, no other income, full standard allowance. An illustration using published 2025/26 rates for England, Wales and Northern Ireland.

Income Tax: taxable income is £40,000 minus the £12,570 allowance = £27,430, all in the basic rate band. At 20%: £27,430 × 20% = £5,486. Class 4 National Insurance: 6% on profits between £12,570 and £50,270, so £27,430 × 6% = £1,645.80. See our guide to Class 4 National Insurance, and why payments matter for your National Insurance contributions record.

The total bill is £5,486 + £1,645.80 = £7,131.80. In a first filing year, with no payments on account already made, the whole bill plus the first payment on account fall due together on 31 January 2027: £7,131.80 + £3,565.90 = £10,697.70 on that one date, with the second £3,565.90 due on 31 July 2027. That 150% January is why freelancers set aside a quarter to a third of every invoice separately.

Records, corrections and your next step

Keep records behind every figure: statements, invoices, receipts. Mistakes are fixable: correct a return within 12 months of the deadline, online or by paper return, waiting 72 hours after filing before amending online. If more tax is due, interest runs from the original payment date. Before the January rush, confirm whether you must file, register by 5 October if new, gather records now, file online well before 31 January. Self Assessment punishes the late, but it is predictable.

This article is general information only, not financial advice.