How HMRC Catches Self-Assessment Errors: What You Need to Know

Worried about HMRC investigating your self-assessment? You’re not alone.

These days, HMRC doesn’t catch tax errors through mysterious means or lucky guesses. They use data-matching systems that are becoming increasingly sophisticated each year.

In 2024/25, these systems helped recover an additional £4.6 billion in unpaid tax. And that doesn’t even touch on the stress and hassle of enduring an investigation that you could have avoided. 

So, without further ado, let’s learn about how HMRC catches self-assessment taxpayers and how to file correctly to avoid issues. 

How Does HMRC Catch Self-Assessment Tax Avoidance?

Connect is HMRC’s data analytics platform, introduced in 2010 and refined continuously since. It processes billions of data points annually from thousands of sources, building comprehensive financial profiles of taxpayers.

The system works through pattern matching. Connect knows what typical financial patterns look like for different professions and income levels. When a tax return doesn’t align with the broader financial footprint, the system assigns a higher risk score.

From 2025, AI has significantly enhanced HMRC’s tax investigation capabilities, including the use of social media analysis. 

As of 2020, over 90% of HMRC investigations now start with Connect alerts rather than random selection or tip-offs.

What Information Does HMRC Receive?

HMRC’s data access is extensive and continues to grow. They receive information automatically from numerous sources:

Financial data:

• Bank account details and transactions from UK and overseas banks
• Payment provider data from PayPal, Stripe and similar services
• Credit reference agency reports on spending patterns and loans
• Credit and debit card transaction data

Government databases:

• Companies House records showing directorships and shareholdings
• Land Registry details of property purchases and sales
• DVLA vehicle ownership records
• Department for Work and Pensions benefits data

Digital platform reporting (mandatory since January 2024):

• eBay, Etsy, Vinted sales and earnings
• Airbnb and short-term rental income
• Amazon and similar marketplace transactions
• Bank account details linked to these platforms

International sources:

• Common Reporting Standard data from over 100 countries
• Offshore bank account details for UK residents
• Foreign tax authority information through automatic exchange agreements

Employment and investment data:

• PAYE submissions from employers
• Pension provider reports
• Investment platform dividend and interest data
• Building society interest payments

HMRC doesn’t access private messages or browsing history, but publicly visible social media content can be used during investigations.

What Triggers HMRC Investigations

Let’s review the core scenarios that trigger HMRC to investigate your tax:

Income and Bank Deposit Mismatches

Money flowing into your bank accounts needs to match what you’ve reported as income. 

Banks and other financial providers automatically send HMRC data about deposits – every payment in, every transfer received. 

The problem arises when bank deposits substantially exceed declared income. Money in your account came from somewhere. Either it’s income you should have declared, or it’s from non-income sources like loans, gifts, inheritance, or withdrawing your own savings. If it’s the latter, you need documentation proving it. 

This catches people who underreport cash earnings, forget side income, or fail to declare freelance work. 

Purchases That Declared Income Couldn’t Support

Lifestyle and spending should realistically match income. This is why HMRC checks social media – think lavish influencer yacht parties while claiming income under the personal allowance. HMRC is coming down on that kind of situation hard. Read our blog on digital creator taxes here.

HMRC can also see property purchases through Land Registry data and vehicle acquisitions through DVLA records. These purchases require substantial funds – deposits, down payments, and outright purchase costs. 

When spending significantly exceeds what declared income can support, it indicates either undeclared income or that legitimate savings/gifts funded the purchases.

The pattern matters too. Consistently spending more than you earn year after year suggests ongoing undeclared income rather than one-off legitimate windfalls.

Overstated Expense Claims

Claiming more in expenses than is reasonable or justifiable for your type of business raises flags. 

The problem may not be that the expenses are completely fabricated – it’s that they’re inflated, mixed with personal costs, or apportioned incorrectly.

Travel expenses are a common area. People claim every mile driven, including personal journeys, or inflate mileage figures beyond what’s realistic for their business activity. A local tradesperson claiming 30,000 business miles annually raises questions. That’s extensive travel for someone working in a limited geographic area.

The same applies to mixed-use expenses. Your mobile phone, internet, vehicle – these get used for both business and personal purposes. You can only claim the business portion, but people often claim 100% when the actual business use is 40%-50%. 

Round Number Expense Claims

The pattern of how expenses are recorded reveals whether they’re actual costs or estimates. Real business expenses are recorded at specific amounts based on actual receipts: £127.83, £2,456.19, £891.45. 

When multiple expense categories are claimed at exactly £3,000, £5,000, or other round numbers, it signals estimation rather than proper record-keeping.

The underlying problem is that estimated expenses aren’t allowable; HMRC requires actual costs supported by receipts.

Round numbers indicate you don’t have the receipts, which means you can’t prove the expenses were actually incurred. During investigations, HMRC will ask for receipts. If you can’t produce them, the expenses get disallowed entirely.

This catches people who’ve kept poor records throughout the year and try to reconstruct their expenses from memory when filing their return.

Extreme Income Fluctuations

Business income naturally varies, but extreme swings need to make business sense. Profit dropping 80% one year then recovering fully the next suggests either volatile trading conditions that should be documented, or inconsistent reporting standards.

Expenses that double year over year without corresponding business growth raise similar questions. Legitimate reasons do exist, such as business expansion, major equipment purchases, and increased activity levels. But the change needs explanation and evidence.

Suspicious Timing and Coincidences

Declared income that suddenly jumps during mortgage applications or loan processes may attract attention. 

When someone reports £30,000 in profit for three consecutive years and then declares £65,000 in the year they apply for a mortgage, HMRC may question whether the increase reflects genuine business growth or is inflated to satisfy lenders.

The same pattern appears with visa applications, benefit claims, or any situation where higher income would be advantageous.

Income that conveniently increases when you need it to, then drops back down afterwards, doesn’t look like normal business trading – it looks like manipulation.

Persistent Losses

Businesses sometimes lose money, especially in the early stages. But reporting losses year after year while continuing to trade full-time doesn’t fit commercial reality. Either the business isn’t viable (so why continue?), or the figures aren’t accurate.

HMRC expects genuine businesses to become profitable within a reasonable timeframe. Persistent losses suggest the “business” might be a hobby being claimed as a trade, or that expenses are overstated to create artificial losses.

Common Mistakes HMRC Catches Every Year

Beyond the major red flags, certain specific errors appear in compliance checks year after year. Most are avoidable with proper record-keeping and understanding of the rules.

The mistakes that repeatedly cause problems:

• Claiming personal expenses as business costs – your regular commute, gym membership, everyday clothes
• Not apportioning mixed-use expenses properly – claiming 100% of your mobile phone when you use it personally too
• Forgetting to declare income that’s been reported to HMRC – bank interest, platform sales, dividends
• Missing entire income sources – side work, freelance payments, rental income
• Poor or non-existent record-keeping – no receipts, estimated figures, incomplete records

The “wholly and exclusively” test is stricter than most people realise, so you need to get this right. While in many cases misattributing smaller expenses will flag as a simple, innocent mistake, you don’t want the hassle. 

If you want to learn more about HMRC tax investigations, head to our guide here.

How to Stay Off HMRC’s Radar

Here’s how to confidently file your self-assessment to stay off HMRC’s radar:

• Keep records as you go. Track income and expenses when they happen, not in January. Photograph receipts immediately. Reconcile bank statements monthly. Reconstructing a year’s worth of expenses from memory is how errors happen.
• Only claim what you can prove. If you’re unsure whether something counts as a business expense, find out before claiming it. When in doubt, get advice or leave it out.
• Declare everything. HMRC receives third-party reports from employers, banks, platforms, and investment providers. Omitting income they already know about guarantees investigation.
• Use actual figures, not estimates. Real expenses are £127.34, £2,891.56 – not round thousands. Precise amounts show proper records.
• Explain significant changes. Sharp increases or declines in income need to make business sense. What looks suspicious is income that increases for mortgage or finance applications, then drops back down.
• File on time even if you can’t pay. Late filing triggers automatic penalties. File by the deadline, then contact HMRC about payment arrangements.
• Fix mistakes quickly. Amend within 12 months if you spot errors. Self-correction looks far better than HMRC finding problems.
• Respond to HMRC promptly. Ignoring correspondence results in penalties and raises suspicion.
• Get professional help for complex situations. Multiple income streams or substantial expenses usually benefit from accountant support that saves more than it costs.

If you’re unsure, uncertain, or confused about a specific case, consider booking a consultation with our team. 

How Double Point Can Help

Understanding HMRC’s systems shouldn’t create anxiety. When you know what HMRC looks for, you can file with confidence.

At Double Point, we help clients file accurate returns that withstand HMRC’s data matching. Our chartered accountants know which expense claims are defensible, what needs supporting evidence, and how to ensure your return is complete and correct from the start.

We can help you set up record-keeping systems that capture everything HMRC expects, categorise expenses correctly, and identify all income sources that need to be declared. Our aim is to ensure HMRC never has reason to question your return.

Stop worrying about HMRC investigations. Book a consultation with Double Point today and file with confidence, knowing your return is accurate, complete, and fully compliant.

Disclaimer: This article is provided for general information purposes only and does not constitute professional tax, legal or financial advice. Tax rules and HMRC’s compliance procedures are subject to change, and individual circumstances vary. Whilst every effort has been made to ensure accuracy at the time of publication, Double Point accepts no liability for any reliance placed on this information. You should always consult with a qualified chartered accountant or tax adviser regarding your specific situation before making any decisions based on this content. HMRC rules can be complex, and what applies to one taxpayer may not apply to another.