Answers · UK 2025/26
What is an HMRC discovery assessment?
An HMRC discovery assessment allows HMRC to raise a tax charge after the normal enquiry window has closed, if they discover income or gains were not declared.
Full answer
Normally HMRC must open an enquiry into a tax return within 12 months of filing. However, under Section 29 TMA 1970, HMRC can raise a "discovery assessment" outside the normal window if they discover that an amount of income or chargeable gains has been insufficient. The time limit for a discovery assessment is: 4 years after the tax year for ordinary errors; 6 years for careless errors or omissions; 20 years for deliberate errors or fraudulent non-disclosure. HMRC must have genuinely "discovered" new information -- not just formed a different view of information already available. Penalties on discovery assessments depend on the behaviour (prompted/unprompted disclosure, careless/deliberate).
This answer is informational only and does not constitute financial, tax or legal advice. Figures are for the 2025/26 UK tax year. See our methodology and sources.