Self-Assessment Tax Return Deadlines (and What Happens If You Miss One)
- Kieran Thwaites
- 1 day ago
- 6 min read
Self-Assessment runs on a handful of fixed dates - and HMRC's penalties for missing them are automatic, steep, and land even if you don't owe a penny.
This guide sets out every deadline that matters, exactly what late filing and late payment cost you, and - if you've already slipped past a date - how to limit the damage.
It's part of our series of simple explainers and sits alongside our Essential Guide to Self-Assessment for the Self-Employed, which covers who actually needs to file. If you'd rather never worry about this again, our post on filing your Self-Assessment early is the companion read.
📋 Table of Contents
📅 The Key Self-Assessment Deadlines
There are only a few dates to remember, but missing any of them has a cost:
Deadline | What's due |
5 October | Register for Self-Assessment (if it's your first year needing to file) |
31 October | Paper tax return |
31 January | Online tax return and payment of any tax owed |
31 January | First payment on account (for the next year) |
31 July | Second payment on account |
The dates relate to the tax year that ended the previous 5 April. So your 2024/25 return had to be filed online and paid by 31 January 2026.
⏰ A Quick Word on Payments on Account
If your tax bill is over £1,000, HMRC usually asks you to pay next year's tax in two instalments "on account" - one on 31 January (alongside your balancing payment) and one on 31 July. They're advance payments towards your next bill, not extra tax. We explain these in full in our guide to payments on account - worth a read if a July demand ever caught you by surprise.
❗ What Happens If You File Late
Late filing penalties are automatic and escalate the longer you leave it. Crucially, they apply even if you owe no tax at all:
The day after the deadline - a fixed £100 penalty. No warning, no grace period.
3 months late - £10 a day for up to 90 days, adding up to £900 on top.
6 months late - a further £300 or 5% of the tax due, whichever is higher.
12 months late - another £300 or 5%, whichever is higher (and up to 100% of the tax in the most serious cases).
Add those up and a return that's a full year late can rack up £1,600 in filing penalties - even on a zero-tax return. Filing and paying are treated as two separate obligations, so you can be penalised on both.
💷 What Happens If You Pay Late
Late payment penalties and interest are charged separately from the filing penalties above:
Interest starts the day after the deadline (1 February) and runs daily until you pay. It's currently 7.75% a year (the Bank of England base rate plus 4%), and HMRC reviews it quarterly - so treat that figure as today's rate (as of August 2026), not a fixed one.
5% of the unpaid tax if it's still outstanding around 30 days after the deadline (early March).
A further 5% at 6 months, and another 5% at 12 months.
The good news: setting up a payment plan with HMRC (see below) before the 30-day point usually stops that first 5% surcharge in its tracks.
🧮 Worked Example: The Cost of Being Six Months Late
Priya owes £2,000 and files and pays her return six months late.
Fixed penalty: £100
Daily penalties (90 days × £10): £900
6-month filing penalty (greater of £300 or 5% of £2,000): £300
Late-payment surcharges (5% at 30 days + 5% at 6 months): £200
Interest (roughly, at 7.75% for six months): ~£78
That's around £1,578 on top of the original £2,000. Notice how most of it - the £1,300 in filing penalties - has almost nothing to do with how much tax she owed. That's why filing something on time matters even more than paying in full.
🔄 What's Changing: Points-Based Penalties
HMRC is gradually replacing the automatic penalty regime above with a points-based system, brought in alongside Making Tax Digital for Income Tax. Under it, each late submission earns a point rather than an instant fine, and a £200 penalty only kicks in once you hit the threshold (two points for annual filers).
The switch applies to MTD for ITSA taxpayers first, from April 2026, with a wider rollout following. Which regime applies to you depends on your income and whether you're in MTD yet — and our tax return service can confirm exactly where you stand.
⚠️ Common Mistakes to Avoid
Assuming "no tax owed" means no penalty. The £100 is triggered by being late, not by owing money.
Forgetting that filing and paying are separate. You can file on time and still be hit with payment penalties, and vice versa.
Missing the 5 October registration deadline in your first year - HMRC can't process a return for someone who isn't registered.
Overlooking payments on account, then being surprised by the July instalment.
Leaving it all to 31 January. HMRC's systems get hammered in late January; a technical glitch is no excuse if you've left no margin. Filing early removes the risk entirely.
🆘 Already Missed a Deadline? What to Do Now
Don't panic - act quickly to stop the meter running:
File as soon as you can. Daily penalties only start at 3 months, so filing now can save you £900.
Pay what you can to reduce the interest and surcharges that build on the outstanding balance.
Set up a Time to Pay arrangement with HMRC before the 30-day mark - this usually prevents the first 5% late-payment surcharge and spreads the cost.
Appeal if you had a reasonable excuse. You can appeal a late-filing penalty (form SA370) within 30 days. Genuine reasons - serious illness, bereavement, or events outside your control - may be accepted; simple forgetfulness usually won't.
If any of this feels overwhelming, that's exactly what we're here for - we can file, appeal and negotiate with HMRC on your behalf.
💬 Frequently Asked Questions
Do I still get a penalty if I owe no tax? Yes. The £100 fixed penalty (and the daily penalties after it) apply to the return being late, regardless of whether any tax is due.
When does interest start on unpaid tax? The day after the payment deadline - 1 February - and it accrues daily until the bill is cleared.
Can I appeal a penalty? Yes, within 30 days, using form SA370. You'll need a reasonable excuse - HMRC looks at the specific circumstances.
What counts as a reasonable excuse? Things genuinely outside your control: serious illness, a bereavement, a fire or flood, or an HMRC service failure. A busy schedule or "I forgot" generally won't qualify.
Can I spread the cost if I can't pay? Often, yes - HMRC's Time to Pay lets many taxpayers pay in instalments. Arranging it early (before 30 days) also helps you dodge the first 5% surcharge.
💡 Key Takeaways
The dates that matter: 5 October to register, 31 October for paper, 31 January to file online and pay, plus payments on account on 31 January and 31 July.
Late filing penalties are automatic and apply even with no tax owed - £100, then £10/day, then tax-geared charges, up to £1,600 over a year.
Late payment is charged separately: 7.75% interest plus 5% surcharges at 30 days, 6 months and 12 months.
A points-based penalty system is phasing in with Making Tax Digital - check which applies to you.
If you've missed a deadline, file straight away, arrange Time to Pay early, and appeal if you have a genuine excuse.
📞 Facing a Deadline (or a Penalty)? Let ASBA Accounting Help
At ASBA Accounting, we keep Crawley's sole traders, landlords and individuals on the right side of every Self-Assessment deadline - and if HMRC has already been in touch, we can file late returns, set up payment plans, and handle appeals for you.
👉 Get in touch today or call us on 01293 525656 for friendly, straightforward advice.
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