Self Assessment deadlines and penalties

The key dates at a glance

The tax year runs from 6 April to 5 April. Your return for a tax year is due the following January.

Note that 31 January 2027 is a Sunday. You can still file online at the weekend, but some payment methods take longer to reach HMRC when a deadline falls on a weekend or bank holiday, so pay by the Friday before to be safe.

Registering

If you've started working for yourself, you must register for Self Assessment by 5 October after the end of the tax year you started in. Then HMRC sends you a UTR. If you're in construction, also register for CIS so contractors deduct 20% rather than 30%. See how to register as self-employed and for CIS.

Filing the return

You can file your return any time from 6 April after the tax year ends. Filing early doesn't mean paying early — the payment deadline is still 31 January — but it does mean:

• you know your bill months in advance;
• any refund, such as a CIS refund, arrives sooner;
• you can reduce your payments on account if your income has dropped.

Paying

Your bill is due by 31 January. If you make payments on account, the second one is due by 31 July.

Late filing penalties

These apply if you send your return late, even if you have no tax to pay or you pay on time.

Someone who files 12 months late with £2,000 of tax due could face £100 + £900 + £300 + £300 = £1,600 in penalties alone.

Late payment penalties and interest

If you pay late, two things happen:

Interest is charged from the day after the deadline on anything you owe. The rate is the Bank of England base rate plus 4%: 7.75% from 9 January 2026. It applies to late payments on account as well as balancing payments.

Penalties apply to the balancing payment if it's still unpaid:

Worked example

Alex owes £4,000 for 2025/26 and pays it on 15 April 2027, 74 days late.

• Late payment penalty at 30 days: 5% × £4,000 = £200
• Interest: about £4,000 × 7.75% × 74 ÷ 365 = £62.85 (assuming the rate doesn't change)
• Total extra: about £262.85

If Alex had set up a Time to Pay arrangement with HMRC before the 30-day point, the penalty could have been avoided, though interest would still be due.

If you can't pay

Contact HMRC as soon as you know. If you owe £30,000 or less, have no other debts or plans with HMRC, and file your return, you may be able to set up a payment plan online yourself. Penalties for late payment aren't charged on amounts covered by a Time to Pay arrangement agreed before the penalty date.

Appealing a penalty

You can appeal if you had a reasonable excuse — something unexpected and outside your control that stopped you meeting the deadline, such as:

• a serious illness or a stay in hospital;
• the death of a close relative shortly before the deadline;
• an HMRC online service failure;
• a fire, flood or theft that stopped you completing your return.

Forgetting, not having the money, or relying on someone else who let you down generally won't count. You normally need to appeal within 30 days of the penalty notice, and fix the problem (file or pay) as soon as you can.

Changing your return

Made a mistake or missed some costs? You can amend an online return within 12 months of the 31 January deadline. For 2025/26 that's by 31 January 2028. After that, you may still be able to claim back overpaid tax with an overpayment relief claim, up to four years after the end of the tax year.

What's changing

From 6 April 2026, people brought into Making Tax Digital for Income Tax use a new points-based penalty system for late submissions, and different late payment penalties. The government has announced that the new system will extend to all Self Assessment taxpayers from 2027/28. Until then, the penalties above apply to everyone else.