Potentially, yes.Expiry of the normal enquiry window generally prevents HMRC from subsequently opening that particular statutory return enquiry. It does not necessarily extinguish every other statutory assessment power.Separate provisions, including discovery assessment powers where their statutory conditions are met, may permit HMRC to assess additional tax after the ordinary enquiry window has closed.Whether HMRC opened an enquiry in time and whether it has a valid later assessment power are different legal questions.

There is no single "how far back" period to which HMRC can assess.Different statutory clocks govern different things. The period for opening an enquiry into a filed return is not the same as the period for making an assessment for tax, and neither should be confused with Schedule 36 rules governing how far back HMRC can go for information and older documents.For the taxes to which the relevant assessment provisions apply, four years is the ordinary period can go back to asses tax, were the facts demonstrate this. Six years can apply where the statutory conditions for careless behaviour are met. A twelve-year period can apply to specified offshore matters involving Income Tax, Capital Gains Tax or Inheritance Tax. Twenty years can apply in cases involving deliberate behaviour or certain statutory failures.The correct period depends upon the tax, the facts and the statutory basis HMRC relies upon.

No.The six-year period is not triggered merely because HMRC disagrees with the return or believes additional tax is due.Where the extended period depends upon carelessness, the loss of tax must have been brought about by careless behaviour of the taxpayer or another person acting on their behalf.The existence of an inaccuracy and the behaviour responsible for it are separate questions."HMRC says the figures are wrong" should therefore not be treated as shorthand for "HMRC automatically has six years".