E1.201A Outline of self assessment provisions
For the latest New Development, see ND.1565.
The overriding principle of self assessment is that the taxpayer is responsible for determining their liability to income tax and capital gains tax by delivering a return giving details of income and gains (including claims for reliefs). These entries are, in the first place, used as the basis of the taxpayer's tax liability without enquiry by HMRC. Only subsequently does HMRC make enquiries into the accuracy of a return or the validity of a claim. This principle is described by HMRC as 'process now—check later' and applies both to tax returns and to claims for relief or repayment.
The self assessment tax return must contain the amount of each source of income or relief claimed, and if the exact amount is not known before the filing date, a 'best estimate' may be included. Once the actual amount is known, an amended return must be filed. For detailed commentary on the use of estimated or provisional figures, see E1.210. The return must also contain a calculation of the tax liability (known as a self assessment), prepared by or on behalf of the taxpayer1 (but see below for an exception to this rule).
A tax account is opened for everyone who is within the self assessment regime, and HMRC issues statements of account (form SA300) periodically.
See also C1.110 regarding the obligation to deliver a return relating to disposals of UK land from 6 April 2019.