What is a Discovery Assessment? When can HMRC make a Discovery? What are the time limits for Discovery Assessment?

Discovery is only possible where a detailed series of conditions are met, see our Subscriber guide Discovery Assessments for full details, examples and current case law and developments.

At a glance

HMRC have the power to re-open a past year of assessment if they discover a past under assessment that results in a loss of tax, and raise a Discovery Assessment.

  • There must be an incomplete disclosure leading to a loss of tax of some kind.
  • The number of years after the end of the tax year that HMRC may go back to depends on the underlying conduct of the taxpayer or their agent in preparing the return.
  • No discovery is possible without evidence: the onus is on HMRC to prove an incomplete disclosure or loss of tax.
  • A future discovery can be avoided by including suitable disclosures on a Return.

Time limits may extend where HMRC is able adopt the Presumption of continuity and it is presumed that the same loss of tax will arise year on year. It is then up to the taxpayer to show that there was no continuity of the fault.

Time limits


Time limit (years)




Incomplete disclosure

Not due to careless or deliberate conduct


A loss of tax

Due to careless conduct


A loss of tax

  • Due to a deliberate action, or
  • A failure to notify liability, or
  • Attributable to a notifiable tax avoidance scheme (DOTAS), a hallmarked scheme or listed scheme, and

The user failed to notify HMRC


Taxpayers are also restricted as to the number of years that they may go back to seek relief from an overpayment after making an error or mistake in a return, see Overpayment relief.

Overview, case law and legislation

See Discovery Assessments (subscriber guide)


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