Key self-assessment dates and how to file

The UK tax year runs from 6 April to 5 April. Understanding this cycle and the deadlines that follow it is crucial to avoid late filing penalties from HMRC.

The Tax Year: 6 April to 5 April

Unlike a calendar year, the UK tax year ends on April 5th. For example, the 2024/25 tax year runs from 6 April 2024 to 5 April 2025.

You must report all the income you received and expenses you incurred between these two dates on your Self Assessment tax return.

Important Deadlines

• 5 October: Deadline to register for Self Assessment if you've never filed one before. (e.g. for the 24/25 tax year, register by 5 Oct 2025).
• 31 October: Deadline to file a paper tax return (though very few people do this anymore).
• 31 January (Midnight): The absolute deadline to file your online tax return AND pay the tax you owe.
• 31 July: Deadline for your second "Payment on Account" (if your previous tax bill was over £1,000).

What are "Payments on Account"?

If your Self Assessment tax bill is over £1,000, HMRC assumes you will owe a similar amount the following year. They require you to pay this in advance in two instalments:

1. 31 January: You pay your actual tax bill for the previous year, PLUS half of the estimated bill for the current year.
2. 31 July: You pay the second half of the estimated bill for the current year.

If you end up earning less than expected, you can ask HMRC to reduce your Payments on Account.

Penalties for Missing Deadlines

HMRC is strict. If you miss the 31 January filing deadline by even one minute, you receive an automatic £100 penalty.

If you are more than 3 months late, you will be fined £10 a day up to a maximum of £900. There are also additional penalties for paying your tax late (calculated as a percentage of the unpaid tax plus interest).