How HMRC Finds Out: The Connect System & Data Intelligence

One of the most common questions we hear from new clients is: “How did HMRC find out?” The honest answer, increasingly, is that HMRC knew before you did. Its Connect data analytics system cross-references information from over 30 sources against what people declare and the gaps trigger investigations. This guide explains what Connect is, what it knows and what to do if HMRC has already found something.

What Is the Connect System?

Connect is HMRC’s sophisticated data analytics platform. It was built specifically to identify non-compliance by cross-referencing information held by multiple agencies, government departments and financial institutions against tax returns and other records filed with HMRC. Think of it as a permanent, automated audit of everyone’s declared position against everything HMRC can find out about them from elsewhere.

Connect processes billions of data points. Its outputs range from the personalised “nudge letters” that HMRC sends to taxpayers it suspects have undeclared offshore assets, to the risk scores that prompt compliance officers to open formal enquiries, to the intelligence that triggers dawn raids and criminal investigations. For the vast majority of people who receive a letter “out of nowhere” from HMRC asking about an overseas bank account or a property they let out, Connect is why.

Where the Data Comes From

HMRC’s Connect system draws on more than 30 data sources. The most important for individuals and small businesses include:

Financial Institutions

• UK banks and building societies reporting interest paid on savings accounts;
• Offshore financial institutions under the Common Reporting Standard (CRS) and FATCA reporting income and account balances to HMRC automatically (see below);
• Card payment processors reporting business turnover to HMRC for businesses accepting card payments;
• Cryptocurrency exchanges , HMRC has received data from UK exchanges and is expanding its crypto data-gathering capability under CARF (the Crypto Asset Reporting Framework) from 2026.

Property and Land

• Land Registry records of property ownership, purchase prices and transfers;
• Stamp Duty Land Tax returns showing property purchases;
• Short-term letting platforms (Airbnb, Vrbo and others) reporting UK host income to HMRC under the Digital Platforms Information Regulations;
• Council tax records showing occupation of properties not connected with declared income.

Government and Company Records

• Companies House records of directorships, share ownership and company accounts;
• DVLA records of vehicle ownership (useful in lifestyle-inconsistency cases);
• DWP records of benefit claims;
• Pension providers reporting pension income;
• Other HMRC data , PAYE records from employers, self-assessment history, VAT returns, CIS subcontractor records.

Social Media and Open Source

HMRC also uses open-source intelligence, including social media accounts and business websites, to identify lifestyle indicators inconsistent with declared income, expensive holidays, vehicles, home improvements and other assets visible online that suggest income not reported on a tax return.

Offshore Data: CRS, FATCA and Automatic Exchange

The most powerful single change in HMRC’s data capability in recent years has been automatic exchange of financial information. Under the Common Reporting Standard (adopted by over 100 countries), financial institutions in participating jurisdictions automatically report to HMRC each year on UK-resident account holders, account balances, income and sales proceeds. HMRC receives this data annually and feeds it directly into Connect.

The United States operates a parallel regime (FATCA) requiring US financial institutions to report on accounts held by US persons and UK institutions to report accounts held by US persons. The UK is both a sender and recipient under these frameworks.

For UK taxpayers with overseas bank accounts, investment portfolios, rental properties or business interests, the data HMRC now receives is comprehensive. Accounts in Switzerland, Channel Islands, Caribbean jurisdictions, Dubai and most major financial centres are reported. The idea that overseas accounts are private from HMRC is, in almost every case, now false. We explain the detail in our resource on the Common Reporting Standard.

Property and Rental Income

Undeclared rental income is one of the highest-risk areas for Connect-triggered enquiries, for a straightforward reason: HMRC can see exactly what properties you own (Land Registry), can cross-check whether you are living there (council tax, electoral roll) and can see income from short-let platforms directly. If you own a second property, are not declaring rental income and HMRC’s data suggests it is being let, a nudge letter or formal enquiry is a very realistic prospect.

The same applies to properties held overseas. CRS data includes foreign rental income. And from 2024, digital letting platforms operating in the UK have been required to report host income directly to HMRC. We cover the detail in our rental property investigation guide and the Let Property Campaign guide.

Business Income and Cash Businesses

For businesses, Connect cross-references declared turnover against card-payment data from processors, bank deposits, purchase invoices visible from VAT returns and sector benchmarks. A restaurant declaring significantly less turnover-per-cover than comparable businesses or a tradesperson showing PAYE income but card terminal receipts suggesting undeclared self-employment, will generate a risk flag.

In cash-intensive businesses, Connect is often the reason HMRC decides to conduct the kind of covert observation investigation described in our professional guide on suppressed-sales investigations.

Finance Act 2024: New Employer Reporting Requirements

The Finance Act 2024 introduced new data requirements that will significantly expand Connect’s reach from the 2025–26 tax year. Employers will be required to report additional information through PAYE, including hours paid to employees. Self-employed individuals and company directors will be required to provide more granular dividend income data and trading start/end dates through self-assessment returns. These measures were designed specifically to give HMRC better data on owner-managed business income, one of the areas where Connect historically had less visibility.

Making Tax Digital and Future Data

The rollout of Making Tax Digital for Income Tax (MTD ITSA), beginning with self-employed individuals and landlords with turnover over £50,000 from April 2026 and extending to those with turnover over £30,000 from April 2027, will give HMRC quarterly digital submissions of business income and expenses. This replaces the annual self-assessment return for many people with a continuous stream of quarterly data, which Connect can monitor in near real-time. The practical implication is that the window between an income omission and HMRC identifying it will narrow dramatically.

Nudge Letters: What They Mean

HMRC sends “nudge letters” (officially called “one-to-many” letters) when Connect identifies a likely discrepancy but HMRC has not yet decided to open a formal enquiry. A nudge letter typically says something like: “Our records suggest you may have overseas income or assets that you have not declared.” It invites you to review your position and amend your returns if necessary.

Nudge letters should never be ignored. They signal that:

• HMRC has specific data about you from a third-party source (often CRS data about an offshore account);
• A formal enquiry may follow if you do not respond;
• Voluntary disclosure in response to a nudge letter, while technically “prompted”, is still treated more favourably than doing nothing and waiting for a full investigation.

We cover nudge letters in detail in our offshore nudge letter guide.

What to Do If HMRC Contacts You

The worst response to any HMRC contact, whether a nudge letter, a formal enquiry opening or a call, is to ignore it or to respond hastily without advice. The right steps are:

1. Do not ignore it. HMRC will escalate if there is no response. Ignoring a nudge letter may convert it into a formal enquiry; ignoring an enquiry opening leads to automatic penalties and potential criminal referral.
2. Do not respond directly without taking advice. What you say to HMRC in correspondence is on the record and can be used against you. A poorly worded initial response can make a manageable situation much worse.
3. Contact a specialist immediately. An adviser who understands HMRC’s investigation procedures can assess what data HMRC is likely to hold, advise on the appropriate disclosure route and manage communications from the outset.

Acting Before HMRC Does: Voluntary Disclosure

If you know you have undeclared income or assets, acting before HMRC contacts you (an “unprompted disclosure”) produces the lowest possible penalty, as low as 0% in some cases for domestic income and significantly lower rates for offshore income than a prompted disclosure following HMRC contact. The main voluntary-disclosure routes are:

• Worldwide Disclosure Facility (WDF) , for undeclared overseas income and assets, see our WDF guide;
• Let Property Campaign , for undeclared rental income from UK residential property, see our Let Property Campaign guide;
• Direct voluntary disclosure , by amending self-assessment returns or writing to HMRC through a tax adviser, for domestic income that does not fall under a specific campaign.

In every case, the time to act is before HMRC writes to you. Once Connect has flagged a discrepancy, the clock may already be ticking.

Frequently Asked Questions