Self Assessment deadline 2027: everything you need to prepare now

The Self Assessment deadline lands on 31 January every year and every year a million people miss it. The work that makes January easy is almost entirely work you can do in October.

The dates for the 2025/26 tax year

The 2025/26 tax year ended on 5 April 2026. The return covering it is due in the window now open:

Who needs to file

You will usually need to file if, in 2025/26, you were self-employed, a partner in a partnership, a landlord, or received income that had no tax taken off it. You may also need to file if you had significant dividend or savings income, made capital gains, are claiming certain reliefs, or were caught by the High Income Child Benefit Charge.

If HMRC has sent you a notice to file, you must file — even if you owe nothing. A return that is not due can be withdrawn, but only by asking HMRC, not by ignoring it.

What missing it actually costs

• £100 the moment you are a day late, whether or not you owe any tax.
• £10 a day after three months, up to £900.
• 5% of the tax due or £300, whichever is greater, at six months — and again at twelve.

Late payment is charged separately: penalties at 30 days, six months and twelve months, plus interest running from 1 February. A return filed on time with the tax paid late is a much smaller problem than the other way round, so if money is tight, file anyway and talk to HMRC about a Time to Pay arrangement.

Payments on account, explained

This is the single biggest source of January shock for people in their second year of self-employment.

If your tax bill is over £1,000 and less than 80% of your tax was collected at source, HMRC asks you to pay towards next year's bill in advance, in two instalments. Each is half of last year's liability.

So in your second January you can be asked for last year's tax plus half of next year's, in the same payment. A £6,000 liability becomes a £9,000 demand. Nothing has gone wrong — but nobody who meets it unprepared enjoys it.

If you know your income has fallen, payments on account can be reduced. Reduce them too far and HMRC charges interest on the shortfall, so it is worth doing with a proper estimate rather than optimism.

The October checklist

Everything below can be done now, and doing it now is why some people find January uneventful:

1. Check you can log in. Find your UTR and your Government Gateway details. Recovering access in late January, when HMRC's phone lines are busiest, is its own small disaster.
2. Pull the bank statements. Full tax year, every account the business touched.
3. Chase the missing paperwork. P60, P45, P11D, dividend vouchers, interest certificates, letting agent statements, CIS deduction statements.
4. Reconcile your bookkeeping. Match every transaction; make a list of the ones you cannot explain while you still remember them.
5. Gather the reliefs. Pension contributions, Gift Aid donations, professional subscriptions, student loan position.
6. Estimate the bill. Even roughly. Knowing the number in October gives you three months to fund it.

One more reason not to leave it

If you are a sole trader or landlord with qualifying income over £50,000, Making Tax Digital for Income Tax now applies to you, which means quarterly updates as well as the annual declaration. Going into that with a year of tidy records behind you makes it routine. Going into it already behind does not.

Read our full guide to MTD for Income Tax →

This article is general guidance based on the rules as we understand them at the date above, not advice for your situation. Tax rules change and thresholds are often updated at fiscal events — check the current position on GOV.UK or speak to us before acting on anything here.