HMRC Self-Assessment: Deadlines, Penalties and Complete Guide for 2026

Self-Assessment is HMRC's system for collecting tax that is not deducted at source through PAYE. If you have untaxed income, gains, or complex tax affairs, you are likely required to file a Self-Assessment tax return. Missing the deadlines produces automatic penalties; understanding the system means you can meet your obligations without stress and without overpaying.

Who Must File a Self-Assessment Return?

You must file a Self-Assessment tax return for a tax year (6 April to 5 April) if, in that year:

• You were self-employed and had trading income over £1,000.
• You were a company director (unless from a dormant company with no transactions).
• Your income exceeded £150,000 from any source.
• You received taxable foreign income.
• You had rental income from property (over £2,500 net of allowable expenses; or any amount if you wish to claim mortgage interest relief).
• You had savings income over the Personal Savings Allowance (£500 for higher-rate taxpayers; £0 for additional-rate taxpayers).
• You received untaxed dividend income above the £500 allowance.
• You need to claim tax relief on pension contributions, charitable donations (Gift Aid higher-rate relief), or trading losses.
• You had capital gains above the annual exempt amount (£3,000 for 2026/27).
• You or your partner received Child Benefit and you or they earned over £60,000 (High Income Child Benefit Tax Charge).
• HMRC has issued you with a notice to file a return.

Note that if you receive untaxed income but owe less than £300, HMRC may be willing to collect it through an adjustment to your PAYE tax code rather than requiring a return — but this is at HMRC's discretion and requires that you notify them of the income in time.

Key Deadlines

The Self-Assessment system operates around a fixed calendar of dates:

5 October: if you are newly required to file a Self-Assessment return (for example, you became self-employed, received rental income for the first time, or first exceeded the income threshold), you must register with HMRC by 5 October following the end of the relevant tax year. Missing this date means HMRC may not issue you a UTR (Unique Taxpayer Reference) in time to file by the January deadline.

31 October: the deadline for filing a paper Self-Assessment return for the tax year ended 5 April. If you choose to file on paper, it must reach HMRC by this date. The vast majority of returns are now filed online, and the paper deadline is rarely relevant.

31 January: the most important date. The deadline for:

• Filing the online Self-Assessment return for the prior tax year (i.e., the return for 2024/25 must be filed by 31 January 2026).
• Paying the balancing payment — the difference between the total tax due for the year and any payments on account already made.
• Making the first payment on account for the current tax year.

31 July: the deadline for the second payment on account for the current tax year.

Penalties for Late Filing

The penalty regime for late filing is automatic — HMRC does not exercise discretion, and ignorance of the obligation is not a defence.

• One day late (1 February or later): automatic £100 penalty. This applies even if no tax is due, and even if the tax has already been paid.
• Three months late (after 30 April): additional daily penalty of £10 per day, up to a maximum of £900 (90 days). This is on top of the initial £100.
• Six months late (after 31 July): further penalty equal to the greater of 5% of the tax due or £300.
• Twelve months late (after 31 January the following year): a further 5% of the tax due or £300 (whichever is greater). In cases of deliberate withholding of information, the penalty can rise to 100% of the tax due.

These penalties accumulate. A return filed 13 months late could face: £100 (day 1) + £900 (daily penalties) + £300 (6-month penalty) + £300 (12-month penalty) = £1,600, plus a percentage of any tax owed. For someone with significant tax due, the percentage-based penalties can be far larger.

Penalties for Late Payment

Separate from the filing penalties, interest and surcharges apply to late payment of tax:

• Interest: HMRC charges interest on all tax paid late, calculated from the date it was due. From 6 April 2025 the rate is the Bank of England base rate plus 4% (it was base rate plus 2.5% until that date). This accrues daily from the payment deadline.
• Late payment surcharge: from 30 days after the payment deadline, a 5% surcharge applies to any outstanding balance. Further 5% surcharges apply at 6 months and 12 months.

Interest and surcharges apply to all unpaid tax including payments on account.

How Payments on Account Work

Payments on account are advance payments towards the following year's tax liability, collected alongside the balancing payment for the current year. They apply where your self-assessment tax liability exceeds £1,000 in a year (after accounting for tax already deducted at source).

Calculation: each payment on account is 50% of the previous year's total tax liability (before any payments on account). Two payments are made: on 31 January and 31 July.

Example: for the 2024/25 tax year, your total income tax and NI liability was £20,000. You have already paid £12,000 through PAYE. Your balancing payment (due 31 January 2026) is £8,000. Payments on account for 2025/26 are also calculated at that point: £10,000 (50% of £20,000 total liability), with £10,000 due on 31 January 2026 and £10,000 due on 31 July 2026.

Reducing payments on account: if you expect your income and tax liability to be lower in the following year than in the current year, you can apply to HMRC to reduce your payments on account. This is called a "claim to reduce payments on account" and can be made through your online HMRC account. If you reduce payments and the actual liability turns out to be higher, interest is charged on the shortfall from the date the payment was originally due.

Foreign Income and the SA106

UK residents must report worldwide income on their Self-Assessment return. Foreign income is reported on supplementary pages SA106. The SA106 includes:

• Foreign employment income.
• Foreign investment income (dividends, interest).
• Foreign property rental income.
• Foreign gains (reported separately on SA108).

Where income has been taxed in the foreign country, a foreign tax credit can be claimed to avoid double taxation. The claim is made on the SA106 and is based on the relevant Double Tax Agreement between the UK and the country of source. Excess foreign tax credit cannot generally be refunded — it can only be used to reduce UK tax on the same income.

Ensuring that SA106 is correctly completed — with the right country codes, gross income, and foreign tax paid — is an area where professional advice is valuable, particularly for individuals with income from multiple jurisdictions.

Making Tax Digital for Income Tax

From April 2026, Making Tax Digital for Income Tax (MTD ITSA) begins to roll out for self-employed individuals and landlords with qualifying income over £50,000. From April 2027, the threshold reduces to £30,000. MTD ITSA requires quarterly reporting of income and expenditure through HMRC-compatible software, replacing the annual return for qualifying individuals.

The annual Self-Assessment return is not abolished — rather, it is transformed into an end-of-period statement and final declaration submitted through the MTD software. The quarterly updates do not create additional tax payment obligations; the payment deadlines remain as described above.

For the majority of clients of this practice, who have complex income sources and significant overseas elements, MTD ITSA will interact with existing tax software and advisory arrangements. Monitoring the timetable and ensuring compliance is in order before the relevant start date is advisable.

How Global Investments Can Help

Global Investments works with internationally mobile professionals, business owners and investors to ensure their UK tax affairs are properly managed — including Self-Assessment compliance, foreign income reporting, and planning around the payment on account system. We work with specialist UK tax advisers and accountants to ensure returns are filed accurately and on time. Contact us if you need assistance with your UK tax return.