21(1)Sub-paragraph (2) applies if an officer of Revenue and Customs discovers that—
(a)an amount of tax that ought to have been assessed under this Part of this Act as tax chargeable on a person for a chargeable period with respect to a single-dwelling interest has not been assessed,
(b)an assessment of the tax chargeable on a person for a chargeable period in respect of a single-dwelling interest is or has become insufficient, or
(c)relief has been given that is or has become excessive.
(2)An officer of Revenue and Customs may make an assessment (a “discovery assessment”) in the amount or further amount that ought in the officer’s opinion to be charged in order to make good to the Crown the loss of tax.
(3)The functions of an officer of Revenue and Customs under this paragraph are also exercisable by the Commissioners for Her Majesty’s Revenue and Customs.