Basis-period transition figures reported for 2023/24 can still cause problems years after the original return was submitted. Traders with accounting dates that did not align with the tax year may have filed using provisional accounts, estimated transition-period profits or an estimated overlap-relief figure. When the final information later becomes available, the correction affects not only 2023/24 but potentially the transition profit being spread through later tax years.
The correction route now depends heavily on timing because the ordinary amendment window for a standard 2023/24 Self Assessment return has already closed.
Why basis-period transition figures still affect later tax years
The 2023/24 tax year was the statutory transition year. A trader with a transition part broadly brought into account the standard-period profit plus profit arising from the additional period running towards 5 April 2024. Available overlap relief was then deducted from the transition element.
Any remaining transition profit was normally spread across five tax years from 2023/24 to 2027/28, unless additional amounts were accelerated.
The basis period transition profit calculation confirms that overlap relief had to be given in 2023/24 and cannot simply be carried forward for use under the new tax-year basis.
A revised overlap figure can therefore change the transition profit itself. Once that figure changes, the amount taxable in 2023/24 and the remaining instalments for 2024/25 through 2027/28 may also require recalculation.
Correcting basis-period transition figures within and outside the amendment window
A taxpayer normally has 12 months from the statutory filing deadline to amend a Self Assessment return. For a standard 2023/24 return, the ordinary amendment deadline was therefore 31 January 2026. By October 2026, that normal amendment route is no longer available in most cases.
The general Self Assessment correction rules distinguish an amendment made within the statutory period from a correction required after it has expired.
Where a genuinely provisional transition or overlap figure was used and the final figure produces lower tax, an overpayment-relief claim may be possible after the amendment window has closed. Such a claim is generally subject to a four-year time limit from the end of the relevant tax year and must satisfy the statutory conditions.
It should not be assumed that every late correction automatically qualifies.
Basis-period transition figures based on provisional information
The distinction between a provisional figure and an estimate intended to be final is important. A provisional figure is used because the precise amount is not yet available and is expected to be replaced. It should represent a reasonable best estimate based on the information available at filing.
HMRC’s current guidance accepts that an overpayment-relief claim can potentially correct an excessive assessment arising from a provisional figure after the amendment window, but the claim may fail where the taxpayer ought reasonably to have known the correct figure before that window closed.
This makes the audit trail important. The original return should ideally have identified the figure as provisional and explained why the final amount was unavailable.
A higher final profit requires a different response
If replacing the provisional figure increases the tax liability, an overpayment-relief claim is obviously not the mechanism. The taxpayer should notify HMRC without delay and provide the revised calculation.
Once the amendment window has closed, HMRC may need to use its discovery powers to formally increase the assessment. Delaying notification after the correct figure becomes known can also affect interest and penalty considerations.
The correction should therefore reconcile the original standard profit, transition profit, overlap relief and amount already brought into charge.
Later-year transition instalments must also be checked
Correcting 2023/24 in isolation can leave later returns inconsistent. If transition profit falls from £50,000 to £35,000 because additional overlap relief is established, the amount carried into subsequent years needs to be recomputed. If transition profit increases, later instalments may also rise.
The correction should be reflected consistently through the taxpayer’s Self Assessment tax returns rather than treating each year as an independent adjustment.
Where historic accounts, partnership changes or missing overlap records make the position uncertain, specialist tax advice should reconstruct the original basis periods before any late claim or disclosure is made.
Disclaimer: This article is provided for general guidance only and does not constitute personal tax, legal or financial advice. The correct tax treatment depends on the specific facts, applicable legislation and HMRC practice at the relevant time. You should contact us for advice tailored to your circumstances before taking or refraining from any action.
