UK Self Assessment Late filing penalties: what they really cost (1 day, 3 months, 6 months)

Missing the Self Assessment deadline is one of those things that can get expensive very quickly, even if you don’t owe much tax. Here’s a clear breakdown of what HMRC charges, when the penalties start, and a few practical tips to avoid (or reduce) them.

This applies to most individual Self Assessment returns.

UK Self Assessment penalties - Key deadlines

• 31 October: paper return deadline (if you file on paper)

• 31 January: online return deadline for the tax year, and usually the payment deadline

• If you miss 31 January, penalties start from 1 February

1 day late: the “automatic” £100 penalty

If your online tax return is filed late (even by one day), HMRC usually charges an automatic:

• £100 late filing penalty

It’s charged even if:

• you have no tax to pay, or

• you’ve already paid all your tax.

3 months late: daily penalties (up to £900)

If the return is more than 3 months late, HMRC can add daily penalties:

• £10 per day for up to 90 days

• Maximum: £900

So at 3 months + 90 days, you can hit £100 + £900 = £1,000 in filing penalties, before anything else.

6 months late: another penalty (the bigger one)

At 6 months late, HMRC may charge an additional penalty. This is typically the higher of:

• £300, or

• 5% of the tax due

So if you owe £8,000, the 5% element alone is £400 (plus previous penalties).

12 months late: another penalty again

At 12 months late, HMRC may charge a further penalty, typically the higher of:

• £300, or

• 5% of the tax due

In serious cases (e.g., deliberate withholding), HMRC can apply higher percentages — but most people fall into the standard rules above.

Important: filing penalties vs late payment penalties (not the same)

Two separate things can apply:

A) Late filing penalties

These are for submitting the return late (the £100, daily penalties, etc.).

B) Late payment penalties + interest

These are for paying tax late. Even if you file the return, paying late can trigger:

• Interest from the due date

• Late payment penalties (often starting at 30 days late, then 6 months, then 12 months)

So you can be hit twice: once for filing late, and once for paying late.

Avoid penalties with AI-speed and human expertise

To avoid UK Self Assessment penalties, TaxHub combines smart automation with real accountants review, so your return gets done quickly and correctly. We cut the admin, keep you organised, and help you file before penalties kick in.

• AI-powered organisation to gather and sort documents faster

• Human-checked accuracy by a qualified UK accountant

• Clear steps and quick support so nothing gets stuck

• On-time filing with a process built around deadlines

Ready to get it done?

Pick your plan and file with confidence