Self Assessment Penalties Explained

A plain-English guide to the current Self Assessment filing and payment penalties. What HMRC can charge, when, and how to challenge it.

Overview

Self Assessment is the system by which self-employed people, landlords, and others with complex tax affairs report their income to HMRC. Most people have an online filing deadline of 31 January (for the previous tax year), though paper returns must be filed by 31 October.

If you miss the deadline or fail to pay your tax bill on time, HMRC can charge a combination of filing penalties, payment surcharges, and interest. These can stack up quickly.

Note for MTD taxpayers

If you joined MTD from April 2026, the old SA penalties still apply to your 2025/26 return (due 31 January 2027). Use the SA calculator for that return. From 2026/27, your returns will be under the new MTD penalty system.

The £100 automatic penalty

The moment your return is one day late, HMRC automatically charges a £100 penalty. This is a fixed charge and applies regardless of whether you owe any tax.

Even if you have a nil return — no income, no tax owed — the £100 penalty still applies if you file even one day late.

This penalty is applied automatically by HMRC’s systems. You will receive a penalty notice, typically a few weeks after the filing deadline.

Daily penalties after 3 months

If you still haven’t filed 3 months after the deadline (i.e. after 30 April for online returns, or after 31 January for paper returns), daily penalties begin.

Days late	Penalty	Maximum
Day 1	£100 automatic	£100
Days 91–180	£10 per day	£900
Day 181 (6 months)	5% of tax due or £300	Whichever is higher
Day 366 (12 months)	Further 5% of tax or £300	Whichever is higher

The daily penalties for days 91–180 are £10 per day for each day the return remains outstanding, up to a maximum of 90 days × £10 = £900 extra.

By the time you are 12 months late, if you had £10,000 of tax due, the total filing penalty could be: £100 + £900 + £500 (5%) + £500 (further 5%) = £2,000, plus payment surcharges and interest.

How payment surcharges work

Separate from the filing penalties, HMRC charges surcharges on the unpaid tax itself if you don’t pay on time. These are charged as a percentage of the unpaid tax.

When charged	Rate	On what
30 days after due date	5%	Unpaid tax at day 30
6 months after due date	Further 5%	Tax still outstanding at 6 months
12 months after due date	Further 5%	Tax still outstanding at 12 months

So on a £10,000 tax bill that remains unpaid for 12 months: total surcharges would be £500 + £500 + £500 = £1,500 on top of filing penalties and interest.

Interest on unpaid tax

Interest accrues on unpaid tax from the day after the payment was due. Unlike penalties, interest cannot be appealed — it’s a statutory charge that compensates HMRC for the time value of money.

The current rate for SA is the Bank of England base rate + 2.5% (7.25% as of May 2025). This rate changes when the base rate changes — always verify the current rate at gov.uk.

Interest is simple (not compound) and is calculated daily: annual rate ÷ 365 × number of days late × amount outstanding.

Reasonable excuse — what qualifies

HMRC will waive a penalty if you had a “reasonable excuse” for the late filing or payment. This is a facts-and-circumstances test — HMRC looks at whether a reasonable person in your situation would have been unable to file on time.

Circumstances that HMRC typically accepts:

Serious or life-threatening illness (yours or a close family member’s)
Bereavement of a close family member around the deadline
A fire, flood, or theft that destroyed your records
HMRC’s own service being unavailable (e.g. their website being down)
Postal delays for paper returns submitted in good time
Unexpected hospitalisation preventing you from filing

Circumstances that HMRC typically does NOT accept:

Not knowing about the deadline
Relying on an agent who failed to file
Being too busy
Having insufficient funds to pay (though this may warrant a payment plan)
A mild illness that wouldn’t have prevented filing

If you have points, you must appeal within 30 days of receiving the penalty notice. The sooner you appeal, the better.

How to appeal a Self Assessment penalty

You have 30 days from the date of the penalty notice to appeal. After that, you can still appeal but must explain why the appeal is late.

Steps to appeal:

Gather evidence supporting your reasonable excuse
Appeal online via your HMRC Self Assessment account, or by post
Include the penalty reference number and your explanation
Attach any supporting documents (medical certificates, death certificates, etc.)

If HMRC rejects your appeal, you can request an independent review, and ultimately appeal to the First-tier Tax Tribunal. An accountant or tax adviser can help with complex cases.

Appeal a tax penalty at gov.uk →

Frequently asked questions

I haven't filed my return for several years — how bad is it?

The penalties can stack up significantly over multiple years. Each missed return generates its own set of penalties. However, HMRC often allows people to regularise their affairs — the sooner you act, the better. Contact an accountant or HMRC directly.

Will HMRC know if I haven't filed?

HMRC has significant data-matching capabilities. If you're registered for Self Assessment, they will chase unfiled returns. They receive information from employers, banks, and other third parties.

Can I file a nil return to avoid the £100 penalty?

Yes. If you have no income to report for the year, you can file a nil return before the deadline to avoid the £100 penalty. You can also contact HMRC to be removed from Self Assessment if you no longer need to file. See: Who must send a tax return →

What's the maximum total SA penalty for one late return?

For a return filed more than 12 months late with significant tax due, penalties can include: £100 (day 1) + up to £900 (daily) + 5% of tax (6 months) + further 5% of tax (12 months) + 5% payment surcharge (30 days) + further 5% (6 months) + further 5% (12 months) + interest. On a £20,000 tax bill, this could exceed £5,000 in penalties alone.

Does the £100 penalty apply if I owe no tax?

Yes. The £100 day-one penalty applies regardless of whether any tax is owed. Even a nil return must be filed by the deadline to avoid this penalty.

What if I can't afford to pay?

Contact HMRC as soon as possible to discuss a Time to Pay arrangement. This can pause penalty charges from the date you contact them. Interest still accrues. HMRC Payment Support →

Can an accountant appeal on my behalf?

Yes. A qualified accountant or tax adviser can appeal HMRC penalties on your behalf, represent you in tribunal proceedings, and often achieve better outcomes through familiarity with HMRC processes.

My accountant filed late — am I still liable for the penalty?

Yes. You remain legally responsible for your own tax return, even if you engage an accountant to file it. However, this may be grounds for a reasonable excuse appeal — document your instructions to the accountant and any failures on their part.

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