Part of our complete guide to Self Assessment.
First: stop panicking, start filing
If 31 January has passed and you haven't filed, the single most important thing you can do is file as soon as possible, not wait for a "better" moment. Every day of delay adds to the penalty, and the return doesn't get any easier to do by putting it off further. For the mechanics of the two separate penalty tracks you're now on, see our guide to late filing vs late payment penalties.
What happens immediately
- A fixed £100 penalty applies automatically, even if you owe no tax and even if you're only a day late
- If tax is owed and unpaid, daily interest starts accruing from 1 February
- HMRC does not usually contact you instantly, the system flags it internally and a penalty notice follows by post, often weeks later, by which time more has accrued
What happens if it drags on
The longer a return stays unfiled, the penalty regime escalates on its own clock:
- 3 months late: £10 a day, up to £900, on top of the initial £100
- 6 months late: a further £300 or 5% of the tax due, whichever is higher
- 12 months late: another £300 or 5% of the tax due, and in cases HMRC considers deliberate withholding, up to 100% of the tax due
Run your own numbers in our late filing penalty calculator to see exactly where you stand today, and how much worse waiting another month would make it.
Key point: HMRC can estimate your tax bill itself (a "determination") if you go long enough without filing, and it's often higher than your real liability would be. You can't challenge a determination properly until you actually file the real return.
If you genuinely can't pay
Filing and paying are judged separately. File the return even if you can't pay yet, this stops the filing penalties from escalating. Then contact HMRC directly about a Time to Pay arrangement, a formal instalment plan, which can reduce exposure to the percentage-based late payment penalties even though interest continues to accrue on the outstanding balance.
If you think you have a reasonable excuse
HMRC will consider cancelling penalties for things like serious illness, bereavement of a close family member, or a documented failure of HMRC's own systems. Being busy, forgetting, or not realising you needed to file are not accepted. An appeal must be made in writing within 30 days of the penalty notice.
Need this sorted quickly? Book a free consultation and we'll get your return filed, assess whether an appeal applies, and set up a payment plan if needed, all before this gets any more expensive.
Frequently asked questions
I missed the deadline by a few hours, does it really matter?
Yes, the £100 penalty is automatic and applies from one minute past midnight on 1 February, there is no grace period for being slightly late.
Will HMRC contact me, or do I need to act first?
HMRC will eventually send a penalty notice, but interest and daily penalties keep accruing in the meantime. File and pay as soon as possible rather than waiting to be chased.
What if I can't pay what I owe right now?
File the return anyway to stop the filing penalties, then contact HMRC about a Time to Pay arrangement for the tax itself. Filing late and paying late are judged separately.
Can I get the £100 penalty cancelled?
Only with a genuine reasonable excuse, such as serious illness or a documented HMRC system failure. You appeal in writing within 30 days of the penalty notice.