Self Assessment 2025/26: Key Deadlines, Late Penalties & How to File

Who needs to file a Self Assessment return for 2025/26?

HMRC collects income tax from the majority of UK taxpayers automatically via PAYE — the pay-as-you-earn system that deducts tax at source from employment income. But for income that falls outside PAYE, or for taxpayers with complex tax affairs, Self Assessment requires you to calculate and report your own tax liability directly.

You need to file a Self Assessment return for 2025/26 if any of the following apply:

1. Self-employment: you were self-employed as a sole trader and your gross trading income exceeded £1,000 (the trading allowance). Even if your net profit after expenses is very small, if gross income exceeds £1,000, you must file.

2. Partnership income: you were a partner in a business partnership. Each partner files their own return reporting their share of partnership income. The partnership itself also files a partnership return (SA800).

3. Rental income: you received rental income of £2,500 or more net of allowable expenses. If your net rental income is between £1,000 and £2,500, HMRC may be able to collect the tax through your PAYE code — contact HMRC to arrange this. Below £1,000 gross, the property allowance applies.

4. Untaxed income: you had untaxed income of £1,000 or more. This includes freelance earnings taxed at source (some are, some are not), interest from savings above the Personal Savings Allowance, and other miscellaneous income.

5. Income above £100,000: if your total income from all sources exceeded £100,000, you must file a return. Above £100,000, the Personal Allowance begins to be withdrawn at £1 for every £2 earned, meaning your effective marginal tax rate on income between £100,000 and £125,140 is 60%.

6. High Income Child Benefit Charge (HICBC): if you or your partner received Child Benefit and either of you had income above £60,000 in 2025/26, you must file. The HICBC claws back Child Benefit at a rate of 1% per £200 of income above £60,000, reaching 100% at £80,000. From 2024/25, the £60,000 threshold applies — it was increased from £50,000 by the Spring Budget 2024.

7. Foreign income: you received income from overseas — a foreign pension, overseas employment income, foreign dividends, rental income from a foreign property — that has not been fully taxed at source.

8. Capital gains: you sold assets realising capital gains above the Annual Exempt Amount (£3,000 in 2025/26). This includes shares held outside an ISA, investment property, business assets, and valuable personal possessions (other than cars and wasting chattels).

9. Company director: you were a director of a limited company, unless the company was a non-trading dormant company and you had no income from it during the year.

10. Pension income: you received pension income that has not been taxed correctly through PAYE — including some private pensions, the State Pension (which is paid gross and taxed through a PAYE coding notice applied to other income), or pension income from a trust.

If you are unsure whether you need to file, HMRC's online checker at gov.uk can help. It is always better to register and file when in doubt — the risk of a penalty for failing to notify HMRC of a tax liability is greater than the administrative cost of filing.

The key deadlines for 2025/26

The 2025/26 tax year ran from 6 April 2025 to 5 April 2026. The deadlines for reporting and paying tax on your 2025/26 income are:

5 October 2026 — Register for Self Assessment If you have not previously filed a Self Assessment return and you have a new liability for 2025/26, you must notify HMRC by 5 October 2026. You do this by registering online via the Government Gateway. HMRC will issue your Unique Taxpayer Reference (UTR) — a 10-digit number used to identify your tax account — typically within 10 working days for online registration. Allow time: UTR letters are sent by post and cannot be expedited. Register as early as possible, ideally in April or May 2026.

31 October 2026 — Paper return deadline If you are filing a paper (postal) tax return rather than filing online, the return must reach HMRC by 31 October 2026. Paper returns are increasingly rare — HMRC encourages online filing and the October deadline is earlier than the online deadline specifically to allow time for manual processing. If you are filing paper, post early. The postmark date does not count — HMRC must receive the return by 31 October.

31 January 2027 — Online return deadline and payment deadline The most important deadline. Your online tax return must be submitted by midnight on 31 January 2027. Your 2025/26 tax liability must also be paid by 31 January 2027. If you owe payments on account (see below), your first payment on account for 2026/27 is also due on 31 January 2027.

31 July 2027 — Second payment on account If you are required to make payments on account, the second payment on account for 2026/27 is due by 31 July 2027.

Understanding payments on account

Payments on account are advance tax payments made twice a year — in January and July — for taxpayers whose previous year's tax bill exceeded £1,000 (and more than 80% of their tax was not collected at source through PAYE).

Each payment on account is half of the previous year's total tax liability. This means HMRC collects tax for the current year before you have filed the return, based on the assumption that income and tax will be similar to the prior year.

Example: If your 2024/25 tax return showed a liability of £6,000, your payments on account for 2025/26 would be £3,000 in January 2026 and £3,000 in July 2026. When you file your 2025/26 return by January 2027, if your actual liability is £7,000, you owe a balancing payment of £1,000 (plus your first payment on account for 2026/27, which would now be £3,500).

This system means the cash flow impact of Self Assessment can be significant in the first year you are required to make payments on account — you may owe both the balancing payment for the prior year and the first payment on account for the current year simultaneously in January.

The Self Assessment penalty system

HMRC operates a structured penalty system for late Self Assessment returns. Unlike some tax systems that are lenient for first offences, HMRC's late-filing penalties begin immediately.

Late-filing penalties

Day 1 (1 February 2027 for 2025/26): A fixed £100 penalty is charged automatically, even if:

• The return is only one day late
• You have no tax to pay (all tax was already paid via PAYE or payments on account)
• You actually paid tax on time — the penalty is for late filing, not late payment

This is an important point many people miss. The £100 penalty is for the act of filing late, regardless of whether there is any underlying tax owed. A zero-tax return filed one day late still generates a £100 penalty.

3 months late (from 1 May 2027): After 3 months of lateness, a daily penalty of £10 per day applies. This continues for up to 90 days, generating a maximum additional penalty of £900. The total fixed and daily penalties after 3 months are therefore up to £1,000.

6 months late (from 31 July 2027): A further penalty of 5% of the tax owed or £300 (whichever is greater) is charged. If you owe £10,000 in tax, the 6-month penalty is £500. If you owe £3,000, it is £300 (the minimum applies).

12 months late (from 31 January 2028): Another penalty of 5% of the tax owed or £300 (whichever is greater). If the failure to file is deliberate and concealed, the 12-month penalty can be up to 100% of the tax owed.

Summary of potential penalties on a £5,000 tax liability:

On top of these filing penalties, late payment penalties and interest apply separately.

Late payment penalties and interest

Late payment penalty (30 days): 5% of the unpaid tax. Late payment penalty (6 months): a further 5% of the unpaid tax still outstanding. Late payment penalty (12 months): a further 5% of the unpaid tax still outstanding.

Interest: HMRC charges interest at the Bank of England base rate plus 2.5 percentage points on unpaid tax from the due date. At current rates (mid-2026), this is approximately 7.5% per annum, compounding daily.

The combination of penalties and interest means that a significant tax liability left unpaid for a full year could cost 17.5% or more in additional charges (5% + 5% + 5% in surcharges, plus 7.5% interest). Filing and paying on time — or, if you cannot pay, contacting HMRC to arrange a payment plan — is always more cost-effective.

How to appeal a Self Assessment penalty

HMRC will waive a penalty if you have a reasonable excuse — a genuine, unforeseen reason why you could not file or pay on time. The test is objective: would a reasonable person in your circumstances, exercising reasonable care, have also failed to meet the deadline?

HMRC accepts as reasonable excuses:

• Serious or life-threatening illness of yourself or a close relative shortly before the deadline
• Bereavement of a close relative or domestic partner shortly before the deadline
• An unexpected hospital stay preventing you from filing
• A fire, flood, or other natural disaster destroying records
• HMRC's own online services being unavailable on or around the deadline (HMRC maintains records of service outages)
• Postal delays or loss of a paper return (you should keep proof of postage)

HMRC does not accept:

• Forgetting about the deadline
• Being too busy with work or business
• Relying on your accountant to file (you remain responsible for your return, regardless of who prepares it — though your accountant may have their own liability to you if they missed the deadline)
• Not receiving a reminder from HMRC (HMRC does send reminders but is not obliged to)
• Not understanding the system or not knowing you needed to file

How to appeal: Submit form SA370 (available on gov.uk) or appeal online via your Government Gateway Self Assessment account within 30 days of receiving the penalty notice. Explain the circumstances clearly and provide any evidence available (medical records, death certificates, hospital discharge letters, screenshots of HMRC service outage notices). HMRC considers appeals on a case-by-case basis. If your appeal is rejected, you can request a statutory review or escalate to the First-tier Tax Tribunal.

How to file your Self Assessment return

Step 1: Register and get your UTR

If you are new to Self Assessment, register at gov.uk via the Government Gateway. For self-employment, use the CWF1 form (available online). For other income types, use the SA1 form. Allow 10 working days for your UTR to arrive by post — longer if you register close to the October deadline.

Step 2: Gather your documents

You will need:

• P60 (from each employer) and P45 if applicable
• P11D or P11D(b) for benefits in kind
• Self-employment income and expense records
• Bank statements (for savings interest)
• Rental income and expense records
• Dividend certificates or broker statements
• Any capital gains calculations
• Pension contribution statements
• Gift Aid donation records (to extend your basic-rate band)

Step 3: File online via HMRC or software

Log in to your Government Gateway account and select "Self Assessment." The online return walks you through each section, calculating your tax liability automatically. Alternatively, use HMRC-recognised third-party software — commercially available options include FreeAgent, QuickBooks, TaxCalc, and many others. Software can be particularly useful if you have complex affairs or are a business owner.

Step 4: Pay your bill

Once submitted, your tax liability is confirmed. Pay via:

• Online banking (faster payments — reference is your UTR followed by "K")
• Debit card via the HMRC payment portal
• Direct Debit (must be set up in advance for the January deadline)
• BACS (allow 3 working days)
• Cheque (allow 5 working days; post to HMRC with a payslip)

Avoid paying by cheque close to the deadline — HMRC must receive and process it by 31 January, not just have it postmarked.

Making Tax Digital for Income Tax: what is coming from April 2026

Making Tax Digital for Income Tax Self Assessment (MTD ITSA) represents the most significant change to self-employed and landlord tax reporting in decades. It replaces the current annual return system with quarterly digital submissions.

Who is mandated from April 2026:

• Self-employed individuals with gross trading income above £50,000
• Landlords with gross property income above £50,000
• Those with combined self-employment and property income above £50,000

What MTD ITSA requires:

• Digital record-keeping using MTD-compatible software (spreadsheets with bridging software also qualify)
• Quarterly updates to HMRC (by 7 August, 7 November, 7 February, and 7 May each year)
• A final declaration at year end (replacing the current SA100 return)

Quarterly updates are summaries, not full accounts: each quarterly update is a summary of income and expenses for that quarter. It does not trigger a tax payment — it just keeps HMRC's estimate of your annual liability current. The tax is still calculated and paid in the annual cycle.

April 2027 extension: those with combined income between £30,000 and £50,000 are expected to be brought into MTD ITSA in April 2027, though this has not yet been confirmed by legislation.

Below £30,000: currently exempt from MTD ITSA. The government has confirmed this group will not be mandated before 2027 at the earliest, and final decisions on whether they will be included at all have not been made.

If you are above the £50,000 threshold, now is the time to choose MTD-compatible software and familiarise yourself with the quarterly reporting cycle. HMRC's own free software tool is available but limited; most accountants recommend commercial software for the quarterly workflow.

Top tips to avoid Self Assessment problems

File early. There is no requirement to wait until January 2027. You can file your 2025/26 return from 6 April 2026 onwards. Filing early eliminates deadline risk, allows you to budget for the tax bill over months rather than weeks, and — if HMRC owes you a refund — means you receive it sooner.

Keep records throughout the year. Self Assessment becomes significantly harder if you try to reconstruct income and expenses from memory or incomplete records at year end. HMRC requires you to keep records for at least 5 years after the 31 January filing deadline for the relevant year. Good record-keeping now prevents problems in an enquiry later.

Consider an accountant if your affairs are complex. A qualified accountant typically costs £200–£600 for a basic Self Assessment return, more for complex situations. The fee is often tax-deductible if you are self-employed, and the cost of an accountant is usually less than the cost of a single late-filing penalty plus interest.

Set money aside as you earn it. Self-employed and landlord income is received gross, without tax deducted. HMRC recommends setting aside 20%–30% of net profit (depending on your expected tax rate) throughout the year so you have the funds available when January arrives.

Double-check your Class 4 NI. Self-employed individuals pay Class 4 NI on profits between £12,570 and £50,270 at 6% (2025/26 rate) and 2% above £50,270. This is in addition to income tax and is often overlooked when estimating a tax bill.

Sources

• HMRC: Self Assessment tax returns
• gov.uk: Register for Self Assessment
• HMRC: Self Assessment penalties
• HMRC: Making Tax Digital for Income Tax
• gov.uk: Appeal a Self Assessment penalty
• HMRC: Pay your Self Assessment tax bill
• gov.uk: High Income Child Benefit Charge