For the UK, the main Self Assessment deadline is 31 January — that's when your online tax return and any tax owed must be with HMRC for the previous tax year (which runs 6 April to 5 April). Miss it and you get an automatic £100 penalty, even if you owe nothing. Filing early is the single easiest way to remove the January stress and any risk of a fine.
The key dates
- 5 October — deadline to register for Self Assessment if it's your first time (so HMRC can issue your Unique Taxpayer Reference in time).
- 31 October — deadline for paper tax returns.
- 31 January — deadline for online returns and to pay the tax you owe for the previous tax year.
- 31 July — second payment on account falls due, if you make them.
What are payments on account?
If your tax bill is over £1,000 and most of it isn't collected at source, HMRC asks you to pay towards next year's bill in advance — in two instalments, on 31 January and 31 July. Each is usually half of your previous year's bill. It catches a lot of people out in their first year, because the January payment can include both the balance for last year and the first payment on account for this year.
The penalties for filing late
Late filing penalties stack up quickly:
- 1 day late — an automatic £100 penalty, even if you have no tax to pay.
- 3 months late — £10 a day, up to a maximum of £900.
- 6 months late — a further penalty of 5% of the tax due (or £300 if greater).
- 12 months late — another 5% (or £300 if greater).
Paying late is charged separately, with interest on the outstanding amount plus further percentage-based penalties the longer it goes unpaid.
How to avoid all of it
- File early. You can submit any time after the tax year ends on 5 April — you don't have to wait until January, and filing early doesn't bring your payment date forward.
- Keep records as you go, rather than reconstructing a year of paperwork under deadline pressure.
- Budget for payments on account so the January bill isn't a shock.
- Ask if you're unsure whether you even need to file — the rules on who must complete a return change over time.
We file our clients' returns well ahead of the deadline, every year — no late-night panic, no penalties. If you'd like your Self Assessment simply taken care of, book a free consultation.
This article is general information, not personal tax advice. Penalty rules and dates can change — check the current position with HMRC or speak to us.