What Happens If You Miss the January 31st Self Assessment Deadline?

For many small business owners, January 31st marks more than just the end of the first month of the year: it’s also the Self Assessment deadline.

But life happens. Busy schedules, unexpected work, or just forgetting can mean the deadline passes without submitting your return. So, what happens if you miss it? And more importantly, how can you minimise the impact?

In this blog, we’ll explain the consequences, the penalties, and what steps you can take if you find yourself in this situation.

1. Automatic Late Filing Penalties

If you miss the 31 January Self Assessment deadline, HMRC automatically issues a £100 fixed penalty, even if you owe no tax.

This might seem small, but it’s just the starting point. If your return remains outstanding after 3 months, further daily penalties of £10 per day can accrue, up to a maximum of £900.

At 6 months late, the penalty increases again, potentially 5% of the tax due or £300, whichever is higher. It’s clear that even a small delay can quickly become costly.

2. Interest and Penalties on Late Payments

Missing the filing deadline often coincides with missing the payment deadline, which is also 31 January.

Interest is charged daily on any unpaid tax from the due date until payment. On top of this, additional penalties may be applied: 5% of the tax unpaid after 30 days, then again at 6 and 12 months.

For example, if your Self Assessment bill was £2,000 and unpaid at 31 January, penalties and interest could easily add several hundred pounds to your bill by mid-year.

3. What Minimise Impact If You've Missed the Deadline

If your return is late, HMRC will usually contact you.

It’s important to respond promptly. Ignoring letters or emails can make the situation worse and lead to higher penalties or enforcement action.

Even if you cannot pay your bill immediately, submitting your return and discussing a payment plan can prevent further charges.

If you’ve missed the deadline, act fast.

1. Submit your return immediately to stop further late filing penalties.

2. Pay as much as you can to reduce interest and penalty charges.

3. Contact HMRC if you need a payment plan or have a reasonable excuse—they may reduce penalties in certain cases.

Working with an accountant can make this process far less stressful and help you plan ahead for future deadlines.

4. Making Tax Digital: Changes Are Coming

It’s also worth noting that the way Self Assessment works is beginning to change.

From April 2026, Making Tax Digital (MTD) for Income Tax will apply to self-employed individuals and landlords with annual business or property income over £50,000. Those earning over £30,000 will follow from April 2027.

Under MTD, affected taxpayers will need to keep digital records, submit quarterly updates to HMRC and file an end-of-year final declaration

While this won’t remove deadlines entirely, it will shift the system toward more regular reporting, making up-to-date bookkeeping more important than ever.

5. Preventing Late Submissions in Future

The best way to avoid penalties is simple: plan ahead.

• Keep records up to date throughout the year.

• Use accounting software to track income and expenses.

• Set reminders for key dates: 31 January and 31 July are critical for most Self Assessment taxpayers.

Many SMEs find that quarterly check-ins with an accountant make the annual Self Assessment process far smoother and less stressful.

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At BCT Accountants, we help small business owners and landlords stay on top of deadlines, minimise penalties, and plan for smoother submissions year after year.

Get in touch today on 01423 431 889 or email office@bctaccountants.co.uk to see how we can help you stay compliant and stress-free.

All information is accurate for the 2025/26 tax year.