Tax Investigation
UK tax glossary · Last reviewed: April 2026
HMRC opens enquiries under s9A TMA 1970 into Self Assessment returns within 12 months of the filing deadline (or within 4 years for innocent errors, 6 years for careless errors, 20 years for deliberate understatement). A full enquiry examines all aspects; an aspect enquiry focuses on a specific issue.
HMRC uses Connect — its data analysis system — to identify discrepancies between tax returns and third-party data (bank records, Land Registry, Companies House, DVLA, online platforms). Unprompted disclosures always attract lower penalties than prompted ones.
Tax investigation insurance (available through professional bodies and commercial policies) covers professional fees during an enquiry. It is particularly worthwhile for the self-employed and landlords. The average cost of defending an enquiry can run into thousands of pounds.
Common questions
What should I do if I receive an HMRC enquiry letter?
Do not ignore it. Reply within the deadline given. Consider engaging a tax adviser or accountant if you have not already. Keep records of all correspondence. Enquiries can take many months to resolve.
Can HMRC investigate me if I have no Self Assessment return?
Yes. HMRC can use its information powers to investigate anyone it believes may have undeclared income, even if they have never filed a return. This is one reason proactive disclosure through campaigns like the Let Property Campaign is advisable.
Related resources
TaxHelper provides general information based on published HMRC rates and guidance. It is not regulated financial or tax advice. For decisions involving significant sums, complex circumstances, or if you are unsure, speak to a qualified accountant or HMRC directly.