If the taxpayer does not have sufficient information to enable them to complete the tax return in the time allowed, they should include either a best estimate or a provisional figure. The taxpayer should not leave a box blank or enter ‘details to follow’ as HMRC will regard this as an incomplete return and the taxpayer will be liable to penalties for late filing. See the Self assessment late filing penalties guidance note.

However, HMRC does not accept that the use of estimated or provisional figures is justified if the taxpayer makes little or no effort to obtain the final figures before the filing deadline and may challenge the completeness of the return if it is suspected that this may be the case (see below).

Note that, in many instances, unincorporated businesses will not need to mark figures as provisional or estimated, provided that they meet generally accepted accounting practice (GAAP). Up to 2023/24, profits of a trade had to be calculated in accordance with GAAP, unless the trader elected for the simplified