Muras Matters: Basis Period Reform


From 6 April 2024, a new ‘tax year basis’ of assessment will apply to the trading profits of unincorporated businesses, such as sole traders and partners, which are subject to income tax. Under the tax year basis, these businesses will be taxed on the profits arising in each tax year (6 April to the following 5 April), regardless of their accounting period end date.

This new ‘tax year’ basis will replace the ‘current year’ basis, where previously the tax for any one tax year was calculated using the profits of the accounting period ending in that year.

Basis period reform therefore effectively breaks the link between the accounting date chosen by a business and when they are taxed on their profits.

The tax year 2023/24 represents a transitional year for the change over from the current year basis of assessment to the new tax year basis.


Basis period reform represents a major change in how the taxable profits of unincorporated businesses are calculated for income tax purposes. Under the new tax year basis, businesses will be subject to tax on their profits arising in the tax year, i.e. from 6 April to the following 5 April. A business which already draws its accounts up to 31 March or 5 April (or any date in between) will be treated as having a year end the same as the tax year, and therefore will be unaffected by the change in basis period.

It should be noted that tax payment and return filing dates will remain unchanged under this basis period reform.

The change in basis period does not mean that a business must draw accounts up to 31 March or 5 April, they are still free to choose the year end date for their accounts. However, for those businesses with a year-end other than 31 March or 5 April (or any date in between), they will need to time apportion profits from two sets of accounts in order to calculate their taxable income for tax years from 2024/25 onwards.


It is likely that some businesses will not have both sets of accounts ready by the time the proprietor needs to file their tax return. For example, a business with a December year end is unlikely to have its accounts for the year ended 31 December 2025 ready by 31 January 2026 filing deadline for the 2024/25 tax return. In these circumstances the business will need to estimate its profit levels for the second set of accounts and include a provisional figure in their tax return. Whilst HM Revenue & Customs (HMRC) do not state specifically how businesses should estimate their profit figure, a business should ensure their estimate is reasonable, or interest and penalties could arise.

The provisional figure will subsequently require correcting for the actual figures and this is will be done by submitting an amended return for that tax year to HMRC. Generally, where a provisional figure is included in a return, it should be corrected without delay once the final figure is known. HMRC have indicated that they will amend their guidance, and allow provisional figures used as a result of basis period reform to be corrected at any time up to the normal amendment deadline. For example, for tax year 2024/25, a business will have until 31 January 2027 to correct a provisional figure. This should allow the amended return to be filed at the same time as preparing the following year’s return.

Transitional year

The 2023/24 tax year will represent a transitional year as the change is made from the current year basis to the new tax year basis. Specific rules apply in this transitional year, which may result in more than 12 months profit being taxed at once. Broadly the profits of a business will be taxed in 2023/24 as follows:

  • The 12 months starting from the end of the accounts period for 2022/23 (referred to as the ‘standard part’) – generally for most businesses this will be their usual year end as taxed under the current year basis; and
  • The period from the end of the standard part to 5 April 2024 (referred to as the ‘transition part’).

The impact of the additional profit bought into account for 2023/24 as a result of the transition part is reduced by:

  • Deducting any overlap relief (from when the business started or from a previous change in accounting date) from the transition part profit; and
  • Spreading any remaining ‘transition profits’ over five years.

For the spreading of the transition profits the default position is that 20% of the transition profits are brought into account in 2023/24, and a further 20% in each of the following four tax years. An individual does have the flexibility to elect to accelerate the amount of transition profits brought into account in any one tax year. They  can choose any additional amount to bring into account. Any remaining transition profits will then be spread equally over the remaining spreading period (subject to any further election in one of those years). This ability to accelerate the amount of transition profits brought into account may be particularly useful if a taxpayer is subject to a lower level of tax than usual in any tax year (for example because they have a large expense or lower income that year). This election must be made on the self-assessment return, and the deadline is one year after the filing date for that return.

The reforms outlined above may lead some businesses to question if they should change their accounting date to 31 March or 5 April (or any date in between). Whilst this will reduce ongoing administrative burdens by removing the need to apportion figures from more than one set of accounts, and the possibility of having to file amended returns to correct provisional figures, it will not remove the need to apply the transitional rules in 2023/24, or prevent additional profits being brought into account. A change in accounting date may affect the timing of when additional tax is paid as a result of the basis period reform for some individuals. However, whether a change in accounting date is suitable or possible is also a commercial decision, and businesses will need to consider the wider pros and cons beyond tax.

The above represents only a general outline of the reforms. Therefore, if you require any more information on any of the points raised above or have any questions on the how the change in basis period may impact your business or tax liability please contact a member of our tax team.

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