The short answer
If you miss the online filing deadline of 31 January, HMRC charges an automatic £100 fixed penalty — even if you don't owe any tax at all. It gets more expensive the longer you leave it, and paying late is penalised separately from filing late. The single best move is to file as soon as you can, even if you can't pay yet, because that stops the filing penalties growing.
The deadlines
For a quick recap (our registration guide covers who needs to register and the full detail):
- Register by 5 October if it's your first year needing to file.
- Paper return by 31 October.
- Online return by 31 January.
- Pay what you owe by 31 January.
- Second payment on account by 31 July, if HMRC has asked you to make one.
What it costs to file late
This is the penalty ladder for a late return (separate from paying late, below). These penalties can stack — the longer the return is outstanding, the more of them apply:
- Missed the deadline: £100 fixed penalty — charged even if you owe no tax or are due a refund.
- More than 3 months late: an additional £10 a day, up to a maximum of £900 (i.e. up to roughly 90 days).
- More than 6 months late: a further penalty of 5% of the tax due, or £300, whichever is greater.
- More than 12 months late: another 5% of the tax due, or £300, whichever is greater.
What it costs to pay late
Paying late is penalised separately from filing late — you can owe both at once, or either on its own:
- 30 days late paying: a penalty of 5% of the unpaid tax.
- 6 months late paying: a further 5% of the tax still unpaid.
- 12 months late paying: another 5% of the tax still unpaid.
On top of those penalties, HMRC also charges interest on the tax you owe for every day it's outstanding, at its published late-payment rate. We haven't quoted a specific interest rate here because HMRC changes it periodically — see the current rate on gov.uk's HMRC interest rates page.
A worked example
This is a simplified illustration, not a calculation of your own bill. Say Priya owes £600 in tax for 2025/26. She misses 31 January, and by the time she files and pays she's about 4 months late on both:
- Filing penalty: £100 fixed penalty, plus she's more than 3 months late, so roughly 30 of those days attract the £10-a-day charge — about £300. Filing subtotal: £400.
- Late-payment penalty: her £600 is also more than 30 days overdue, so a 5% penalty applies — 5% of £600 = £30.
- Running total: about £430 in penalties — before any interest HMRC adds on the unpaid £600 at its published late-payment rate.
Your own numbers will be different. Use the side-hustle tax calculator to estimate what you actually owe, then measure how many days late you are against the ladders above.
What to do right now
- File even if you can't pay yet. Filing and paying penalties are separate — filing on time (or as soon as possible) stops the filing-penalty ladder, even if you need more time to pay.
- Work out what you owe. Use the free calculator to estimate your tax so you know the number you're dealing with.
- Not registered yet? See our guide to registering for Self Assessment.
- Have a reasonable excuse? You can appeal a penalty on gov.uk if something outside your control — like a serious illness or bereavement — made you late.