What to have ready before the 31 January Self Assessment deadline

The Self Assessment deadline feels a long way off in October, which is exactly why it catches so many people out. The return for the 2025 to 2026 tax year (6 April 2025 to 5 April 2026) is due online by 11:59pm on 31 January 2027, and the tax you owe is due on the same day. That leaves a little under fifteen weeks, and the part that takes time is rarely the return itself. It’s pulling a year of income and expenses into a shape you can trust.

Here are the dates that matter, what happens if you miss them, and a sensible order to work in.

The dates

• 5 October 2026: the deadline to tell HMRC you need to file a return, if you haven’t filed one before or didn’t need to for 2024 to 2025. If you’ve missed it, register now rather than waiting.
• 31 October 2026: the deadline for a paper return. Almost everyone files online, so this one rarely applies.
• 30 December 2026: if you’d like what you owe collected through your tax code, you need to file by this date (and be eligible for that route).
• 31 January 2027: the online return and the payment, both by 11:59pm.

What a missed deadline costs

Filing late is charged first. There’s an immediate £100 penalty, then £10 a day once you’re three months late (capped at £900), then a further 5% of the tax due or £300, whichever is greater, at six months, and the same again at twelve months. Paying late is charged separately: 5% of the unpaid tax at 30 days, six months and twelve months, plus interest on what you owe. Filing on time but paying late still counts as late payment, so the two deadlines are worth treating as one.

Check what 31 January will actually ask you for

For most sole traders the January payment is bigger than the tax on that year’s profit alone. Once you’re past the first year, HMRC asks for payments on account, which are advance payments towards the next bill, due on 31 January and 31 July. Each one is half of the previous year’s bill, and they apply unless your last bill was under £1,000 or you paid more than 80% of it outside Self Assessment.

The way the dates line up, 31 January 2027 can include both the balancing payment for 2025 to 2026 and the first payment on account for 2026 to 2027. If your profits went up, the balancing payment will be larger too. It’s worth working out the full figure early so the date doesn’t arrive with a surprise attached.

A reasonable order to work in

1. Gather the records now. HMRC expects you to keep records of your business income and expenses. If you don’t prepare separate accounts, that means the tax year, 6 April to 5 April. Cash basis is the default, so you record income and expenses when the money moves, not when an invoice is raised.
2. Fill the gaps while you still remember them. A receipt photo with no context is hard to use in January. A line of note about what it was for, added in the same week, makes the difference between a record you can claim against and one you have to guess at.
3. Sort the grey areas into one list. Anything where you aren’t sure it’s allowable (a phone, a laptop, part of a home cost) belongs on a short list you can take to your accountant, instead of being decided in a rush on the last weekend.
4. Ask your accountant for their cut-off. If you use one, they’ll often want your records before HMRC’s deadline, not on it.
5. File early if you can. A return filed in December leaves a clear month to arrange the payment.

How long to keep everything

HMRC’s rule is to keep your records for at least five years after the 31 January submission deadline. For this return, that means until at least 31 January 2032.

Where this leaves you

None of this needs doing in a single sitting, and the people who find January painful are usually the ones who left the gathering until January. A few evenings in November, with the receipts and bank statements in front of you, will do most of the work.

That gathering step is what Tidysum is built around. You photograph a receipt when you get it, it suggests an HMRC category and explains why, you confirm it, and at the end of the year you can export a categorised record to send to your accountant. It won’t file your return for you, but it means the records you start January with are already in order.