Missing a Self Assessment deadline means an automatic penalty, even if you owe no tax. Whether you're a sole trader, a landlord or a company director, here are the dates that matter — and how Making Tax Digital is about to change the rhythm.

The core Self Assessment deadlines

For a given tax year (which runs 6 April to 5 April):

  • 5 October — deadline to register for Self Assessment if it's your first year or your circumstances have changed. Miss this and you risk a failure-to-notify penalty.
  • 31 October — deadline to file a paper tax return (midnight).
  • 31 January — deadline to file online, and to pay the tax you owe (midnight).

So for the 2025/26 tax year, the online return and payment are due by 31 January 2027. For 2024/25, they were due by 31 January 2026.

Payments on account

Many people forget that 31 January can carry two payments:

  • The balancing payment for the tax year just filed.
  • Your first payment on account towards next year (usually 50% of the previous year's bill).

The second payment on account is then due on 31 July. Payments on account catch a lot of first-time filers off guard, so budget for them.

What happens if you're late

  • £100 fixed penalty the moment you miss the 31 January filing date — even with nothing to pay.
  • After 3 months, £10 a day can start adding up.
  • Further penalties and interest accrue at 6 and 12 months, plus interest on late-paid tax.

Filing early doesn't mean paying early — you can submit in April and still pay in January — so there's no downside to getting it done.

How Making Tax Digital changes this

From April 2026, Making Tax Digital for Income Tax starts replacing the annual return for sole traders and landlords with income over £50,000 (then £30,000 from 2027 and £20,000 from 2028). Instead of one January deadline, affected taxpayers keep digital records and send quarterly updates plus a final declaration. If that's you or your clients, the old once-a-year scramble is on its way out — see our guide to MTD for Income Tax.

Never chase a deadline again

At BookEnu we build reminders around each client's Self Assessment, VAT and year-end dates, so nothing is left to the last week of January. Keeping records digital and current all year is also exactly what MTD will require — so getting ahead now pays off twice. Apply for early access if you'd like to try it on your own clients.

General information, not tax advice, and deadlines can change — always check current HMRC guidance for a specific situation.