Self Assessment Tax Return Deadlines Explained (2026/27)
Missing a Self Assessment deadline can lead to unnecessary penalties and interest from HMRC. The good news is that, with a little planning, it's easy to stay on top of your obligations.
Who Needs to Complete a Self Assessment Tax Return?
You may need to complete a Self Assessment tax return if you:
Are self-employed.
Receive rental income from property.
Have untaxed income.
Need to report Capital Gains Tax.
Receive certain investment or overseas income.
Are a partner in a business partnership.
Need to claim specific tax reliefs or allowances.
Have been asked by HMRC to complete a tax return.
Important: A high salary on its own no longer automatically means you need to complete a Self Assessment tax return. If all of your income is taxed through PAYE, you may not need to file a return unless HMRC requests one or you have other income to report.
If you're unsure whether you need to file, it's worth checking before the deadline to avoid potential penalties.
The Key Self Assessment Deadlines
For most taxpayers, there are just three dates to remember. If you are completing a tax return for the tax year 6 April 2025 to 5 April 2026, your key deadlines are:
5 October 2026 – Register for Self Assessment (first-time filers only).
31 January 2027 – Submit your online tax return and pay any tax due for the 2025/26 tax year. This is also the deadline for your first Payment on Account for 2026/27 (if applicable).
31 July 2027 – Make your second Payment on Account (if applicable).
Most Self Assessment tax returns are now submitted online. A separate 31 October deadline applies only to paper tax returns. If you're eligible to have tax collected through your PAYE tax code, you'll normally need to file online by 30 December.
What Are Payments on Account?
Many people are surprised when their first Self Assessment tax bill is larger than expected.
This is because HMRC may ask you to make Payments on Account towards your next year's tax bill.
Who Do Payments on Account Apply To?
Payments on Account usually apply if:
Your Self Assessment tax bill is more than £1,000, and
Less than 80% of your tax has already been collected through PAYE or another source.
If your Self Assessment tax bill is £1,000 or less, or most of your tax has already been deducted before you receive it, you will not normally have to make Payments on Account.
A Simple Example
Imagine you've completed your 2025/26 Self Assessment tax return. The return calculates that you owe £8,000 in tax.
Because your tax bill is over £1,000, HMRC assumes you will earn a similar amount next year and will ask for payments in advance:
31 January 2027: Balancing Tax due for 2025/26 - £8,000
31 January 2027: First Payment on Account for 2026/27 - £4,000
Total due on 31 January 2027- £12,000
31 July 2027: Second Payment on Account for 2026/27 - £4,000
Although your tax return calculated a tax bill of £8,000 for the 2025/26 tax year, HMRC is also asking you to pay half of your estimated tax for the following year in advance.
When you later submit your 2026/27 tax return, HMRC compares the tax you've already paid in advance with the amount actually due:
If you've paid too much, you'll normally receive a refund or have the overpayment set against future tax.
If you've paid too little, you'll simply pay the remaining balance.
Can Payments on Account be reduced?
If you expect your tax bill to be lower than the previous year, you may be able to reduce your Payments on Account. Take care not to reduce them too far, as HMRC may charge interest if you underpay.
Craigerne Tip: We encourage clients to prepare their tax return well before January. Knowing your tax bill early gives you time to budget for any Payments on Account and avoids last-minute surprises.
What Happens If You Miss a Deadline?
Missing a Self Assessment deadline can quickly become costly. HMRC enforces a strict, escalating penalty regime for late filing and late payments—even if you ultimately owe no tax at all.
Here is exactly how the penalties stack up if you miss the 31 January deadline:
1 day late: An immediate £100 automatic penalty applies the moment you miss the midnight deadline.
3 months late: HMRC charges an additional £10 per day for up to 90 days, adding a maximum of £900 to your bill.
6 months late: A further penalty of 5% of the tax due or £300 (whichever is greater) is added.
12 months late: Another 5% of the tax due or £300 (whichever is greater) is applied. In serious cases, the fine can be up to 100% of the tax due.
Late Payment Penalties and Interest
Filing your tax return on time is only part of the process—you also need to pay any tax due by the deadline.
If payment is made late, HMRC will charge interest on the outstanding amount from the day after the payment deadline. Depending on your circumstances and which penalty regime applies, HMRC may also charge late-payment penalties if the tax remains unpaid.
If you're struggling to pay, it's important to contact HMRC as soon as possible. In many cases, agreeing a Time to Pay arrangement before penalties arise can help reduce or avoid some late-payment penalties.
What Information Will You Need?
Having your paperwork ready makes completing your tax return much easier.
Depending on your circumstances, you may need:
Employment income (P60 or P45).
Self-employment income and expenses.
Rental income and expenses.
Bank interest.
Dividend income.
Pension contributions.
Gift Aid donations.
Capital Gains information.
Keeping organised records throughout the year can save considerable time when your tax return is due.
Common Mistakes We See
Some of the most common issues include:
Leaving everything until January.
Forgetting to register for Self Assessment.
Missing allowable business expenses.
Forgetting rental or dividend income.
Losing receipts and records.
Underestimating the first tax bill because of Payments on Account.
Getting advice early can often save both tax and stress.
Top Tips
Register early if this is your first tax return.
Keep digital records throughout the year.
Don't wait until January to gather your paperwork.
Set money aside regularly for your tax bill.
How Craigerne Accountancy Can Help
At Craigerne Accountancy, we help individuals, landlords and business owners complete accurate Self Assessment tax returns on time.
We can help you:
Prepare and submit your tax return.
Claim all allowable expenses and tax reliefs.
Understand your tax bill before it's due.
Help you budget for Payments on Account.
Deal with HMRC on your behalf if required.
Whether it's your first tax return or you've been filing for years, we'll make the process straightforward, accurate and stress-free.