UK Tax Return issues? filing a Self Assessment tax return should feel straightforward. Many people do it every year. Yet thousands of people get it wrong and face penalties.

More than 11.5 million taxpayers filed their Self Assessment tax return by the 31 January deadline for the 2024–25 tax year. However, HMRC data confirms that over 1.1 million people missed the deadline, instantly triggering an automatic £100 fine.

These are not rare tax filing errors. They are common, costly, and totally avoidable.

This guide details the real mistakes people make when filing their UK tax return and how you can avoid them before HMRC flags your account.

Why Mistakes Hurt You (The 2026 Reality)

Most people think tax errors only cost time. In 2026, they cost far more than that. HMRC’s interest rate for late payment has hit 7.75% (as of Jan 2026). This means every day you delay paying the correct amount, your debt grows significantly faster than inflation.

HMRC charges:

• Automatic fines for late returns (£100 minimum).
• Daily penalties if you delay filing for 3 months or more.
• Interest on unpaid tax (currently 7.75%).
• Extra penalties for “careless” or “deliberate” errors (up to 100% of the tax due).

HMRC does not need to prove intent. If the return contains mistakes like misreported income or wrong relief claims—you are liable. Avoiding mistakes is about clarity and accuracy, not complexity.

How to Know If You Must File UK Tax Return?

Before you start, ask one question: Do you have to file a Self Assessment tax return?

You must file for the 2024/25 tax year (ending 5 April 2025) if:

• You are self-employed and earned over £1,000 (gross).
• You received rental income (even from Airbnb).
• You sold assets like shares or property with capital gains above £3,000.
• You received untaxed income (e.g., dividends over £500).
• You or your partner claimed Child Benefit and one of you earned over £60,000 (High Income Child Benefit Charge).

If HMRC sent you a notice to file, you must file. Ignoring it will trigger penalties even if you owe no tax.

 

7 Mistakes People Make When Filing Their UK Tax Return

Mistake 1: Missing the Deadline

The most frequent and costly mistake is a late return. The UK deadline for online filing is 31 January after the end of the tax year.

• For the 2024–25 tax year: The deadline is 31 January 2026.

Miss this date by one minute, and HMRC fines you £100 immediately. If you are 3 months late, daily fines of £10 begin.

• Fix: Start early. File before the deadline, even if you cannot pay the full bill yet. Filing on time stops the late filing penalty; you can then arrange a “Time to Pay” plan for the bill.

Mistake 2: Leaving Out “Hidden” Income

HMRC’s “Connect” system receives data from banks, employers, and investment platforms. If your return ignores this data, it flags an enquiry. Commonly missed income sources:

• Side Hustles: Earnings from Vinted, eBay, or freelance gigs over £1,000.
• Bank Interest: Interest earned on savings (outside of an ISA).
• Dividends: The tax-free allowance is now just £500. Many investors forget to declare amounts above this.
• Crypto: HMRC is actively sending “nudge letters” to crypto traders. Every trade is a taxable event.
• Fix: Gather all your income documents (P60, bank statements, crypto reports) before you start. Cross-check your figures.

Mistake 3: The High Income Child Benefit Trap

This is the most common error for employed high earners. If you or your partner earn over £60,000 and claim Child Benefit, you must pay some of it back via the High Income Child Benefit Charge (HICBC).

• The Mistake: People assume it only applies to the claimant. It actually applies to the highest earner in the household, even if the other parent claims the benefit.
• Fix: Check your (and your partner’s) adjusted net income. If it is over £60k, register for Self Assessment.

Mistake 4: Claiming Expenses Without Proof

Expenses lower your tax bill, but HMRC expects proof—not guesses. You can only claim expenses that are wholly and exclusively for business. Commonly rejected claims:

• Home Office: Claiming a flat percentage of rent/mortgage without a proper calculation.
• Clothing: Claiming for a “business suit” (this is not allowed; only protective gear or uniforms are).
• Travel: Claiming commute costs (home to permanent office) as business travel.
• Fix: Keep digital copies of receipts for 5 years. Use the “Simplified Expenses” flat rates if you don’t have detailed bills.

Mistake 5: Capital Gains Errors (The 60-Day Rule)

Since October 2021, if you sell a UK residential property (that isn’t your main home), you must report and pay the tax within 60 days of completion.

• The Mistake: Many people wait until their annual tax return to report this. By then, they are months late and face penalties.
• Fix: If you sold a property, check if you need to file a separate “Capital Gains Tax on UK Property” return immediately. Then, include the figures on your SA108 form later.

Mistake 6: Misunderstanding PAYE vs Taxable Income

People with employment income sometimes think HMRC has already taxed everything. It hasn’t always. If you have Benefits-in-Kind (like private medical insurance or a company car) that are not fully payrolled, or if your tax code was wrong, you may owe more tax.

• Fix: Check your P60 and P11D forms. Don’t assume HMRC’s pre-filled figures are perfect.

Mistake 7: Not Paying on Time (The 7.75% Cost)

Filing on time is one thing. Paying on time is another. If you file correctly but pay late:

• HMRC charges 7.75% interest (current rate).
• A 5% penalty is added if the tax is 30 days late.

Fix: Estimate your tax early. If you can’t pay, set up a Time to Pay arrangement before the deadline to avoid the 5% penalty.

The One-Page Checklist Before You Submit

Before you press submit, verify you have:

1. P60 and P45 (from all employers).
2. Bank Interest Statements (for all accounts).
3. Dividend Vouchers (check the £500 allowance).
4. Expense Receipts (organized by category).
5. Rental Income Records (income vs expenses).
6. Capital Gains Calculations (check the £3,000 allowance).
7. High Income Child Benefit Check (is income >£60k?).

What to Do If You’ve Already Filed Incorrectly?

Mistakes don’t lock you into penalties forever. HMRC allows you to amend your return for up to 12 months after the filing deadline.

However, amending it incorrectly can trigger an enquiry. Common amendment problems include:

• Amending the wrong tax year.
• Forgetting to update the supplementary pages.
• Triggering a refund that HMRC later rejects.

This is where professional help protects you. Tax Returns Accountants review your submission line by line. We identify the error, correct it properly, and submit the amendment in the right format. Most importantly, we help you fix errors before HMRC contacts you. Early action often reduces penalties to zero.

Still unsure if your return is right? Don’t wait for the brown envelope to arrive. Let Tax Returns Accountants review your return today. Fill out our service request form to secure your peace of mind.

Questions People Ask – FAQs

1. What happens if I submit after the deadline?
   You receive an automatic £100 penalty immediately. After 3 months, daily penalties of £10 begin. You also pay interest on any unpaid tax. 
2. Can I file even if I don’t owe tax?
   Yes. If HMRC asked you to file, you must file to avoid penalties, even if the balance is £0. 
3. How long should I keep receipts? Self-employed individuals must keep records for at least 5 years after the filing
   deadline. Individuals with only personal income (like dividends) should keep them for 22 months. 
4. Do I need to declare foreign income?
   Yes. If you are a UK tax resident, you must declare worldwide income (unless you are non-domiciled and claim the remittance basis, which is complex).

      5. Can I amend my tax return after submission?
Yes. You have 12 months from the filing deadline (e.g., until Jan 31st 2027 for the 2024/25 return) to make corrections online.