Self Assessment Tax Return UK 2026
UK self assessment 2025/26 return: key deadlines, allowances, payment-on-account explained, and the 6 mistakes that trigger HMRC penalties. A plain-English guide.
The self assessment tax return UK 2026 deadline is 31 January 2027 if you file online, or 31 October 2026 if you still use paper. Miss the online date by a day and HMRC charges £100, even if you owe nothing.
The numbers people search for are the ones nobody writes down properly. So this guide goes through 2025/26 self assessment in plain English — who has to file, the allowances you can claim, how payments on account work, and the six mistakes that trigger penalties every year.
Key 2026 deadlines
The tax year in question is 2025/26 — it ran from 6 April 2025 to 5 April 2026. You file the return for that year in the months that follow.
| Date | What is due |
|---|---|
| 5 October 2026 | Register for self assessment if it is your first time |
| 31 October 2026 | Paper return for 2025/26 |
| 30 December 2026 | Online return if you want tax under £3,000 collected through PAYE |
| 31 January 2027 | Online return, balancing payment, and first payment on account for 2026/27 |
| 31 July 2027 | Second payment on account for 2026/27 |
You can confirm these on the HMRC page Self Assessment tax returns — deadlines.
The 5 October date is the one new filers miss most. If you started self-employment in March 2026 and only register in November, HMRC charges a failure-to-notify penalty even though you are still inside the filing deadline.
Who needs to file a self assessment
HMRC runs PAYE for most employees, so a return is needed only when income or status falls outside that net. You must file a self assessment tax return UK 2026 if any of the following applied during 2025/26:
- You were self-employed as a sole trader and earned more than £1,000 in turnover.
- You were a partner in a partnership.
- You received untaxed income above £2,500 — rent, tips, commission, dividends from outside an ISA, foreign income.
- You were a company director with untaxed income.
- You had total income above £150,000.
- You earned more than £50,000 and your household claimed Child Benefit (High Income Child Benefit Charge).
- You sold an asset and owed Capital Gains Tax.
- You wanted to claim higher-rate relief on pension contributions or charity Gift Aid above the basic-rate amount.
Even if HMRC sends you a return, you should check whether you actually need to file. The opposite is also true — many people start a small side business and assume PAYE covers them. If your side income passes the £1,000 trading allowance, register by 5 October the following tax year.
The £1,000 trading allowance covers gross income, not profit. If you earned £2,000 selling on Etsy with £1,500 of costs, you still need to register for self assessment — the trading allowance is breached on turnover.
Allowances you can claim for 2025/26
Most people pay too much tax simply because they forget the standard allowances. The table below is the short version — there are more for specific trades.
| Allowance | Amount (2025/26) | Who claims it |
|---|---|---|
| Personal allowance | £12,570 | Most UK residents (tapered above £100,000) |
| Dividend allowance | £500 | Anyone with dividend income |
| Personal savings allowance | £1,000 basic / £500 higher | Anyone with bank interest |
| Trading allowance | £1,000 | Side income up to this level is tax-free |
| Property allowance | £1,000 | Rental income up to this level |
| Marriage allowance | £1,260 | Non-taxpayer spouse can transfer to basic-rate spouse |
| Capital Gains annual exempt amount | £3,000 | Capital gains up to this level |
If you are a sole trader, you can choose between the trading allowance (£1,000 against gross income, no expense records needed) or claiming actual business expenses. The trading allowance suits low-cost service businesses. Anyone with stock, equipment, or mileage usually does better claiming real expenses.
Common expense categories for sole traders
- Office costs — phone, internet, software, stationery.
- Travel — fuel, parking, public transport, but not your commute.
- Use of home — flat-rate amounts from £10 to £26 a month, or actual costs apportioned.
- Stock and raw materials.
- Bank charges and accountant fees.
- Marketing and website costs.
HMRC publishes the simplified expenses figures on Simplified expenses if you're self-employed. The flat rates often beat actual records once you factor in the time it takes to keep them.
Payment on account explained
This is the part that floors first-time filers. If your tax bill for the year is over £1,000, HMRC asks for two payments on account towards next year's bill on top of the current year's balancing payment.
A worked example. You file your 2025/26 return online in January 2027 with a tax bill of £6,000. On 31 January 2027 you pay:
- The £6,000 balancing payment for 2025/26.
- Plus £3,000 — half of an estimated £6,000 bill for 2026/27 — as the first payment on account.
That is £9,000 in one go, and many self-employed people are not warned. The second payment of £3,000 then falls on 31 July 2027. When you file the 2026/27 return in January 2028, the £6,000 already paid is credited, and any difference is settled as a balancing payment or refund.
If you know your 2026/27 income will be lower (you closed the business, or had a year off), you can ask HMRC to reduce the payments on account. Do not over-reduce — they will charge interest if your final bill is higher than the reduced figure.
How to register for self assessment
If 2025/26 is your first year of self assessment, you register with HMRC by 5 October 2026. The process takes two steps and HMRC posts a code, so do not leave it until October.
- Go to Register for Self Assessment and complete the online form (SA1 for non-business, CWF1 for sole traders).
- HMRC sends a Unique Taxpayer Reference (UTR) by post — usually within 10 working days.
- You then enrol for online services at your Government Gateway account. HMRC posts an activation code, again about 10 days.
- Once activated, you can file your return online.
Allow three weeks total. If you wait until January and the UTR letter is delayed, you will not be able to file on time.
Six mistakes that trigger HMRC penalties
- Filing late even when no tax is owed. The £100 fixed penalty applies whether your bill is £0 or £50,000. Daily £10 penalties start three months after the deadline, capped at £900. After six months, HMRC adds 5% of the tax owed or £300, whichever is higher.
- Forgetting the second payment on account in July. The first payment in January is unavoidable because it falls with the return. The July payment is silent — no reminder, no fresh return. Many people pay it months late and incur interest.
- Mixing personal and business bank accounts. It is not illegal, but it makes records hard to defend in an HMRC enquiry. Open a free business current account and run all business income and expense through it.
- Claiming round-sum amounts that look made up. If you put down £500 for office expenses and £500 for travel with no record behind them, HMRC will notice on review. They expect itemised records, even if monthly totals are used in the return itself.
- Missing the registration deadline. Failure to notify by 5 October triggers a penalty based on the tax due, which can be 30% or more if HMRC concludes you deliberately stayed out of the system.
- Underestimating Class 2 and Class 4 NI. Sole traders pay Class 4 NI at 6% above £12,570 and 2% above £50,270, plus a flat Class 2 if profits are above £6,725. NI runs on top of Income Tax and is collected through the same return — many forget the rate when budgeting.
Penalties can be appealed if you have a reasonable excuse (a death in the family, serious illness, an HMRC system failure). "I forgot" or "my accountant did not get back to me" do not pass the test.
Records HMRC expects you to keep
Keep records for at least five years after the 31 January filing deadline. For 2025/26 that means until at least 31 January 2032. The HMRC list:
- Sales invoices and receipts.
- Purchase invoices and receipts.
- Bank statements (business and any personal account where business money flowed).
- Mileage logs if you claim vehicle costs.
- Capital purchase records for equipment.
Cloud accounting software (FreeAgent, Xero, QuickBooks) handles most of this automatically. HMRC has confirmed that digital records meet the requirement, so paper-shoebox systems are no longer needed.
Frequently asked questions
When is the self assessment deadline for 2025/26?
Paper returns: 31 October 2026. Online returns and balancing payment: 31 January 2027. First payment on account for 2026/27 also due 31 January 2027.
Who needs to file a self assessment?
Sole traders earning over £1,000, company directors with untaxed income, landlords with rental income over £1,000, anyone with income over £150,000, plus people claiming certain reliefs (e.g. pension contributions over the allowance).
What is payment on account?
If your tax bill is over £1,000, HMRC asks for half of next year's estimated tax with your January return, and the other half by 31 July. This spreads the tax bill but catches first-time filers by surprise.
What happens if I file late?
£100 automatic penalty after 31 January, even if no tax is owed. Daily £10 penalties after 3 months. 5% of unpaid tax after 6 months. Penalties stack — late filing costs add up fast.
Can I claim expenses without receipts?
HMRC expects records for at least 5 years after the 31 January following the tax year. Bank statements help, but receipts are the gold standard. For mileage, keep a log.
This article is general guidance, not personal tax advice. Speak to a qualified accountant before acting on it.