Self Assessment Deadlines and Penalties (2025/26): The Dates You Can't Miss

Self Assessment penalties are automatic, escalating, and entirely avoidable. The whole system runs on a handful of dates — miss them and it costs you, even if you owe no tax. Here are the deadlines and exactly what being late costs.

The key dates

What	Deadline
Register (first-time filers)	5 October after the tax year
Paper tax return	31 October
Online tax return	31 January
Pay the tax you owe	31 January
First payment on account	31 January
Second payment on account	31 July

The tax year runs 6 April to 5 April, and the all-important date is 31 January — when your online return and your payment are both due. We explain the system in Self Assessment explained.

The penalties for filing late

These are automatic and stack up:

Up to 3 months late: £100 fixed penalty — even if you owe no tax
Over 3 months: £10 per day, up to £900
6 months late: a further 5% of the tax due (or £300 if greater)
12 months late: another 5% (or £300 if greater)

A return left a year unfiled can therefore rack up well over £1,000 in penalties before you’ve paid a penny of actual tax.

The penalties for paying late

Filing on time isn’t enough — you also have to pay on time. Late payment carries its own penalties, separate from filing:

5% of the unpaid tax at 30 days
5% again at 6 months
5% again at 12 months
Plus interest charged daily from the due date

So both filing and paying late are punished — and they’re cumulative.

Appealing a penalty

You can appeal if you have a “reasonable excuse” — a serious illness, a bereavement, or a genuine HMRC system failure, for example. You’ll usually need to file the outstanding return first. Everyday reasons like “I forgot” or “it was confusing” generally won’t succeed.

Don’t forget payments on account

A common shock for first-time filers is being asked to pay payments on account — advance instalments towards next year’s bill — on top of what you owe. It can double your January payment, so it’s worth understanding in advance. See payments on account explained.

The simplest way to never miss a deadline

Penalties are 100% avoidable — they only happen when returns are left late. The easiest insurance is to file early. Our Self Assessment service prepares and files your return well ahead of 31 January, confirms your bill in good time, and makes sure you know about any payments on account — so late-filing penalties are simply never on the table.

Frequently asked questions

When is the Self Assessment deadline?

The main deadline is 31 January following the end of the tax year — that's when your online return and any tax due must be submitted and paid. Paper returns are due earlier, by 31 October. If you're filing for the first time, you must register by 5 October after the tax year you need to report.

What is the penalty for filing a tax return late?

You get an automatic £100 penalty if your return is up to 3 months late, even if you owe no tax. After 3 months, daily penalties of £10 a day apply (up to £900). At 6 months and 12 months, further penalties of 5% of the tax due (or £300 if greater) are added each time. Late payment also attracts separate penalties and interest.

What's the penalty for paying tax late?

Late payment of the tax itself carries penalties separate from the filing penalties: 5% of the unpaid tax at 30 days, again at 6 months, and again at 12 months — plus interest charged daily on the outstanding amount from the due date. So paying late is costly even if you filed on time.

Can I appeal a Self Assessment penalty?

Yes, if you have a 'reasonable excuse' — such as a serious illness, bereavement, or a genuine technical failure — you can appeal a penalty with HMRC. You generally need to file the outstanding return first and explain the circumstances. 'I forgot' or 'it was too difficult' aren't usually accepted.

How do I avoid Self Assessment penalties?

Register early, keep your records current through the year, know your tax bill in advance, and file well before 31 January rather than on the night. The simplest insurance is to have an accountant prepare and file your return early — penalties for late filing simply don't happen when the work is done ahead of time.

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Reviewed by Provense Accountants

Written and reviewed by our team of qualified accountants (AAT-regulated). This guide is general information, not personal tax advice — book a free consultation for advice on your situation.