Self Assessment Deadline: The Complete Guide to 31 January

The key self assessment deadlines

The deadline that matters to most people is 31 January: online tax returns for the previous tax year must be filed by midnight, and any tax you owe must be paid on the same date. But it is only one of four dates on the self assessment calendar.

• 5 October — register for self assessment if you need to file for the first time
• 31 October — deadline for paper tax returns
• 31 January — deadline for online returns, payment of your tax bill, and your first payment on account
• 31 July — second payment on account, if payments on account apply to you

What happens if you miss the deadline?

HMRC issues an automatic £100 penalty the moment the deadline passes — even if you owe no tax at all, and even if you are due a refund. From three months late, daily penalties of £10 apply for up to 90 days (another £900). At six months and again at twelve months, further penalties of £300 or 5% of the tax due (whichever is greater) are added.

Interest also runs on unpaid tax from 1 February, and separate late-payment penalties apply at 30 days, six months and twelve months. A return that is a year late can easily cost £1,600 or more in penalties before any tax is counted.

Payments on account: the bill people forget

If your last self assessment bill was over £1,000 and less than 80% of your tax was collected at source, HMRC asks you to pay next year’s tax in advance in two instalments — 31 January and 31 July — each of half your previous year’s bill. Many first-time filers are caught out in January by owing one-and-a-half times what they expected: the balance for last year plus the first instalment for this year.

If you know your income has fallen, you can apply to reduce your payments on account — but reduce them too far and HMRC charges interest on the shortfall. This is exactly the kind of judgement call an accountant makes for you.

Already missed it? Do this now

File as soon as possible — penalties escalate with time, so every week matters. If you had a genuine reason (serious illness, bereavement, HMRC system failures), you may be able to appeal the penalty with a reasonable excuse claim. And if you cannot pay the bill in full, HMRC’s Time to Pay arrangement lets many taxpayers spread the cost over monthly instalments — arranged before the debt escalates.

Bayar Hughes & Co regularly takes on clients mid-crisis: we file the outstanding return, handle the penalty appeal where there are grounds, and set up a payment plan. The sooner you act, the more options you have.

How to make next January stress-free

The filing window opens on 6 April — you can file ten months early and still not pay until 31 January. Early filing means you know your bill months in advance, refunds arrive sooner, and your accountant has time to find every allowance instead of racing a deadline.

Send us your records after the tax year ends in April, and your return is done by summer. Fixed fee, agreed up front — talk to Bayar Hughes & Co in New Eltham, London SE9.