Self Assessment Tax Return Deadline 2026: Everything You Need to Know

The Self Assessment tax return for the 2025-26 tax year (6 April 2025 to 5 April 2026) has two separate deadlines depending on how you file — and a third deadline for paying the tax you owe. Miss any of them and HMRC will issue automatic penalties.

Here is everything you need to know about the deadlines, what happens if you miss them, and how to get your return filed well ahead of time.

Key Dates at a Glance

Paper Filing Deadline: 31 October 2026

If you file a paper SA100 return by post, HMRC must receive it by 31 October 2026. Note that this is the date HMRC must receive it — not the date you post it. Allow at least five working days for delivery if you are posting close to the deadline.

Paper filing is increasingly uncommon, but it remains an option for those without internet access. Bear in mind that paper returns require you to do your own tax calculations, whereas online filing calculates your liability automatically.

Online Filing Deadline: 31 January 2027

The online Self Assessment deadline for the 2025-26 tax year is 31 January 2027. This covers SA100 returns filed through HMRC's own Government Gateway service or via third-party software such as Filabl that integrates with HMRC's API.

January is consistently HMRC's busiest period. Their systems can be slow, and waiting until the last minute creates unnecessary risk. HMRC does not accept "the website was slow" as a reasonable excuse for a late submission.

Tax Payment Deadline: 31 January 2027

Your balancing payment — the remaining tax owed for 2025-26 after any payments on account — is also due on 31 January 2027. This is separate from your return deadline, but they fall on the same date.

If your tax bill for 2025-26 is more than £1,000 and less than 80% of your tax was collected at source (e.g. through PAYE), HMRC will also require you to make payments on account towards your 2026-27 bill:

• First payment on account: 31 January 2027 (50% of the 2025-26 bill)
• Second payment on account: 31 July 2027 (another 50%)

This means your January 2027 payment could be 150% of your 2025-26 tax liability if you are making payments on account for the first time. Plan your cash flow accordingly.

Penalties for Late Filing and Late Payment

HMRC's penalty regime for Self Assessment is automatic and cumulative:

Late filing penalties

• 1 day late: £100 automatic penalty, regardless of whether you owe any tax
• 3 months late: An additional £10 per day, up to 90 days (maximum £900)
• 6 months late: An additional 5% of the tax owed, or £300, whichever is higher
• 12 months late: A further 5% of the tax owed, or £300, whichever is higher. In serious cases of deliberate withholding, this rises to 70–100% of the tax due.

Late payment penalties

• 30 days late: 5% of unpaid tax
• 6 months late: A further 5% of unpaid tax
• 12 months late: A further 5% of unpaid tax

Interest also accrues on unpaid tax from the payment deadline at the HMRC late payment rate, which is currently linked to the Bank of England base rate plus 2.5 percentage points.

Tips to File Early and Avoid the January Rush

• Register for Self Assessment now if you have not already. HMRC can take up to 10 working days to issue your Unique Taxpayer Reference (UTR). Do not leave registration until December.
• Gather your records as soon as the tax year ends. The 2025-26 tax year ended 5 April 2026. Your P60, dividend vouchers, rental income records, and bank statements are all available now.
• File in summer. Filing between May and September means HMRC's systems are quieter, you get your refund sooner if you are owed one, and you know your exact tax liability months before you have to pay it.
• Set aside tax as you earn. A common rule of thumb for sole traders is to set aside 25–30% of net profit in a separate savings account throughout the year.
• Use software that calculates your liability automatically. Manual calculation is error-prone. MTD-ready software pulls in your income and expenses and computes the correct figure.

Who Needs to File a Self Assessment Return?

You must file a Self Assessment return for 2025-26 if any of the following apply:

• You were self-employed (sole trader) and earned more than £1,000
• You were a partner in a business partnership
• Your total income exceeded £150,000
• You received untaxed income of more than £2,500 (e.g. rental income)
• You received income from abroad
• You claimed Child Benefit and your household income exceeded £60,000 (High Income Child Benefit Charge)
• You are a company director (unless your only income is a salary taxed through PAYE)
• You received a P800 from HMRC saying you owe more tax