HMRC « Deadline Day »: The simple mistake on your 2026 form that triggers an automatic audit.

One tab with HMRC, another with online banking, a phone buzzing with reminders. You think you’ve been squeaky clean this year. Then — right as you hit submit — one tiny box decides your fate: interest. The smallest mistake there is the one that trips HMRC’s automatic checks, stalls refunds, and opens the door to an audit trail you never wanted.

The kettle is cooling on the counter when the site finally loads. It’s late on 31 January 2026, and the Self Assessment portal has that midnight-glow aura only stress can make. Somewhere between PAYE figures and expense notes, your cursor pauses over “Untaxed UK interest.” The number feels annoyingly trivial. A few quid, maybe. You type 0 to move on. Clean, tidy, quick.

The page flickers. The threshold between relief and risk is thinner than it looks. And that box has its own secret rules.

The £0 interest trap: the tiny box that trips HMRC’s robot checks

Here’s the crux. Millions of returns won’t fall over on big, juicy fraud flags. They stumble on the “nothing to see here” moments. The most common? Tapping in £0 — or leaving blank — for untaxed bank interest on your 2025/26 return. Banks have been paying decent rates again, so that box is rarely truly zero. HMRC already has a feed of savings data from UK banks and building societies. Your “zero” and their “some” don’t match.

On Deadline Day, this mismatch is like a flare. The Connect system, HMRC’s risk engine, cross-checks your return with employer RTI data, P60s, P11Ds and bank reports. When your form says £0 interest but your bank reported, say, £23.46 over the tax year, your risk score bumps up. Nothing dramatic at first glance — until a refund is held, a nudge letter lands, or an enquiry is auto-opened for clarification. Tiny box, big consequences.

Think of Sam, a graphic designer on PAYE with a small side hustle. Last year, they dutifully typed 0, assuming “the bank sorts the tax.” HMRC’s data showed £48.77 interest across two savings pots. Their repayment — modest but needed — was paused for checks. Sam was fine in the end, but it took three letters and two months. The kicker? Interest had ticked up silently in late 2025 when promotional rates edged higher. You don’t always notice a handful of pounds. HMRC’s algorithms do.

The logic isn’t sinister. When pre-filled data in HMRC’s systems doesn’t align with your declared totals, automated rules fire. Most just trigger an extra look or a request for evidence. Some route into fuller enquiries. The risk isn’t about being “caught.” It’s about needless friction — delays, admin, and the dread that hovers in the inbox. The fix is boring, exacting and oddly calming: get the number right in the first place.

How to get the right number — and keep your return out of the risk lane

Start with your banking apps. Look for “interest paid” statements for each account. Many apps show calendar-year totals by default; you want the tax year: 6 April 2025 to 5 April 2026. Export transactions, filter for “interest,” and sum them for that date range. If the app is clunky, your online banking usually lets you download a CSV. Add ISA interest? Don’t — it’s tax-free. Add children’s accounts? Only if the money is yours and the interest isn’t from a Junior ISA. It takes 15 minutes. Worth every second.

Now cross-check with your HMRC Personal Tax Account. HMRC may display savings interest figures it has received, but those feeds can lag or exclude a smaller provider. Treat it as a sense-check, not gospel. If your tally is a little higher than HMRC’s view, declare yours. If it’s lower, pause and retrace steps. On a tous déjà vécu ce moment où the number you thought was obvious turns slippery. Let it be slippery now, not after submission.

People trip over the year boundary, every time. Banks love calendar years. HMRC taxes on tax years. Another slip: mixing up dividend allowance and savings allowance. For 2025/26, the Personal Savings Allowance stays at £1,000 for basic rate, £500 for higher rate, and nil for additional rate. The dividend allowance sits at £500. If you’re within the allowance, you still report the income. Let’s be honest: nobody does this every day.

Round numbers are another red flag. **Round numbers scream estimate**. Put the exact pennies if you have them. If you truly can’t find a precise figure, note your method in your records — screenshot, spreadsheet, whatever keeps you covered. And if you’ve switched banks mid-year, chase both sets of statements. One missing £3.14 can hold a refund hostage while HMRC waits for you to reply to a letter you didn’t expect.

“Robots don’t care about intent. They care about patterns. If your pattern doesn’t match the data we already hold, a gate comes down.”

Here’s a quick box to keep beside you on Deadline Day:

• Use 6 Apr 2025 to 5 Apr 2026 dates for interest totals.
• Exclude ISA interest and Premium Bonds prizes — they’re tax-free.
• Include tiny amounts from old or dormant accounts.
• Keep exact pence. Avoid “£0” unless you’re absolutely certain.
• If in doubt, check the HMRC Personal Tax Account for a reality check.

Why a small mismatch lands loud — and what it says about HMRC’s 2026 risk radar

HMRC’s Connect system is trained on mismatches. PAYE vs P60, benefits vs P11D, bank interest vs Self Assessment — it hunts for the gaps. The idea isn’t to punish. It’s to prioritise where their limited human time goes. So the cleanest way to dodge an audit trail isn’t crafty. It’s ordinary. Accurate figures in the small boxes deflate your profile into normality.

There’s also a cultural shift. Since rates climbed in 2024–2025, more everyday savers drifted over the Personal Savings Allowance without noticing. Add a regular saver pot here, a prize draw account there, a short-term fix that matured in autumn. You end up with surprise taxability. If your return says “nothing to see,” HMRC’s feed says “something small but real,” and your refund claim is large enough to matter, the system hesitates.

*The truth is, boring beats brave on Deadline Day.* You don’t need exotic tactics. You need the exact line for interest, the right year for dividends, and a quiet confidence that your numbers match the world as HMRC already sees it. File once, sleep well, move on. The alternative is emails, envelopes, and the slow thud of an enquiry you didn’t need.

There’s a second layer hidden in plain sight. If you complete the employment section by copying a payslip instead of the P60, the total can be off by pennies. Those pennies link back to income tax paid and can contradict the interest picture that pushes you over or under an allowance. **Small inconsistencies chain together**. One “£0” in savings and one “rounded” salary figure can look like sloppiness to a machine.

Then there’s the repayment magnet. Big refunds make systems cautious. If you’re claiming back an overpayment while also declaring £0 savings interest in a year when banks were generous, your profile gets under the microscope. That’s fine if you’re right. It’s a headache if you’re simply rushing. You can avoid the dance with one careful pass.

What about people with nothing but ISAs? You still confirm the zero is real by checking statements. If all your savings are in ISAs and Premium Bonds, your untaxed interest likely is zero. But confirm that an old easy-access account didn’t cough up £1.87 in June. That coin looks invisible now; it’s not invisible to HMRC. **The smallest numbers carry the loudest signals** when they collide with a claim.

You can also shave risk in minutes. Open your dividend statements from platforms and confirm the tax year totals. Dividend allowance shrank in recent years; more people tick the threshold without realising. If you put “0” for dividends but your platform reports £212, it’s the same story: a gentle but firm pause from the system, a request for backup, and a spring you didn’t plan to spend scanning PDFs.

None of this needs to be scary. It’s honest arithmetic, done once with the right dates and the right totals. The relief when you hit submit with precise pennies is oddly tangible. Your return becomes boring in the best way — and boring almost never gets audited.

There’s a quiet upside to all this. The Personal Tax Account is becoming more useful as a cross-check, not a crutch. If you start with your own records, then glance at HMRC’s view, you’ll often catch tiny differences before they become letters. That’s the whole game: catch your own small mismatches first.

And if your numbers disagree with HMRC’s display, keep a short note: “Interest from X Bank for 6/4/25–5/4/26 totalled £23.46. Statement attached.” It’s the kind of dull documentation that makes any enquiry vanish quickly. No drama. Just a tidy, human explanation that matches your tidy, human maths.

One last thing another taxpayer wishes they’d known: if you open or close accounts mid-year, your interest can post in chunks that don’t line up with your memory. A closing statement in September might include an accumulation you forgot. Fetch it now. Your future February will silently thank you.

Keep the conversation going

You might read this and think, do pennies really matter? They do in a system that’s pattern-driven and swimming in third-party data. When a few pounds of unreported interest can hold up a three-figure refund, the cost of guessing is weirdly high. Share the nudge with a friend who’s filing late — the person you know who has five banking apps and a “sort it later” folder.

It’s not about being perfect. It’s about being specific. The 2026 Self Assessment is a snapshot of your year. The snapshot is truest when you record the small stuff. If you’ve ever felt talked down to by tax talk, treat this as the opposite: a neighbour’s tip at the door, right before they head out with the bins.

Robots run the gates, but people still write the letters. Save them for someone else. Save yourself a February of mild panic. The fix is simple enough to do while the kettle reheats.

FAQ :

• What exactly triggers the “automatic audit” on interest?When your return shows £0 (or blank) for untaxed UK interest but HMRC’s bank data shows a positive amount, their system flags the mismatch for checks.
• How do I find the right interest figure?Download statements for each account, filter transactions marked “interest,” and add them up for 6 Apr 2025 to 5 Apr 2026. Exclude ISA interest and Premium Bonds prizes.
• My bank shows a calendar-year total. Can I use that?No. Convert it to the tax year by summing transactions that fall between 6 April and the following 5 April. Many apps let you export and filter by date.
• Do I still report interest if I’m within my Personal Savings Allowance?Yes. The allowance affects how much tax you pay, not whether the income is reportable on your Self Assessment.
• What if HMRC’s view of my interest differs from mine?File the figure you can evidence and keep your workings. If HMRC queries it, share your statements or CSV and a short note explaining your method.