Self Assessment Deadlines and Penalties: What Being Late Really Costs
Every Self Assessment date that matters, what each missed one costs, how payments on account work, and what HMRC will actually accept as a reasonable excuse.
The penalties for Self Assessment are automatic. No one at HMRC decides whether to charge them, and they apply whether you owe £20,000 or nothing at all. Here is the full calendar, what each missed date costs, and the one thing worth doing if you are already late.
The dates
| Date | What is due |
|---|---|
| 5 October | Register for Self Assessment if this was your first year of self-employment or rental income |
| 31 October | Paper returns |
| 30 December | Deadline to ask for tax under £3,000 to be collected through your tax code |
| 31 January | Online return, the balancing payment, and the first payment on account |
| 31 July | Second payment on account |
What filing late costs
| How late | Penalty |
|---|---|
| 1 day | £100, fixed, regardless of what you owe |
| 3 months | £10 per day for up to 90 days — a further £900 |
| 6 months | 5% of the tax due, or £300 if greater |
| 12 months | Another 5% or £300, and more where HMRC judges the delay deliberate |
A year late on a modest bill therefore reaches roughly £1,600 in filing penalties alone, before any late payment charge or interest. This is why filing the return matters even when you cannot pay: the two penalties are separate, and filing stops the larger one.
Paying late is charged separately
Penalties of 5% of the unpaid tax apply at 30 days, six months and twelve months. Interest runs from the due date throughout. If you cannot pay, contact HMRC and ask for a Time to Pay arrangement — agreed before the penalty date, it usually prevents the late payment penalties, though interest still applies.
Payments on account: the bill that surprises people
If your bill is over £1,000 you pay next year's tax in advance in two instalments. In your first profitable year that means settling the year just gone and paying half of the next one on the same day — a bill 50% larger than the tax you actually owe for the year.
You can apply to reduce them if you know income has fallen, but reduce them below what turns out to be due and HMRC charges interest on the difference. Reduce them on evidence, not on optimism.
If you are already late
- File the return. Daily penalties keep accruing until you do. This is the single most valuable thing you can do today.
- Pay what you can. Interest is charged on the balance, so a part payment costs less than none.
- Ring HMRC about Time to Pay if the full amount is not possible.
- Appeal within 30 days if you have a genuine reasonable excuse.
Getting help with this
AJH Accountants handles Self Assessment returns for clients across the UK. We file the return, tell you what you can claim, and deal with HMRC on your behalf, so nothing depends on you remembering a deadline.
See how our Self Assessment returns service works, or send us a quotation request and we will call you back. You can also call the office on 01204 840303.
Originally published at ajhaccountant.co.uk/blog/self-assessment-deadlines-penalties-uk.