Basis Period Reform

Basis Period Reform is a fundamental shift in how self-employed sole traders and business partners are assessed for Income Tax. It permanently aligns taxable business profits with the UK tax year, which runs from 6 April to 5 April.

The purpose of the reform is to simplify financial reporting and eliminate the complex historical "overlap profit" framework.

The Old Rules: Current Year Basis

Under the historical system, your tax liabilities were dictated by your business's individual accounting year-end, rather than the statutory tax year. For example, if your chosen accounting year ended on 31 December 2022, those 12 months of profit were taxed as a block in the 2022/23 tax year.

While this granted accounting flexibility, it created significant administrative friction when businesses first started or changed their accounting dates. It frequently created "overlap profits," where a business's opening months of profit were accidentally taxed twice, creating a hidden tax credit that could only be reclaimed when the business ceased trading or changed its year-end date.

The New Rules: Tax Year Basis

The old system has been completely replaced by the Tax Year Basis. All self-employed businesses and partners are taxed strictly on the actual profits earned within the tax year itself, regardless of their chosen accounting year-end date.

If your business accounting year does not naturally align with the tax year (meaning it does not end between 31 March and 5 April), you are required to perform a time-apportionment calculation across two separate sets of accounts to find your true tax year figure.

Example for the 2026/27 Tax Year:

If your business maintains a 31 December accounting year-end, your taxable profit for the 2026/27 tax year (6 April 2026 to 5 April 2027) must be mathematically split on a daily or monthly basis from two separate accounting periods:

• 270 days of profit from your accounts for the year ending 31 December 2026, plus
• 95 days of profit from your accounts for the year ending 31 December 2027.

Administrative Impact: Because the accounts for the later period (ending December 2027) will rarely be completed before the January filing deadline, HMRC permits you to enter provisional estimates on your tax return. These estimates must then be corrected via a formal amendment once your final accounts are drawn up.

The Spreading of Transition Profits

The 2023/24 tax year served as the transitional bridge to move businesses over to the new system. During that year, non-aligned businesses had to pull forward extra months of profit to catch up to the April tax year-end. To prevent double taxation, all remaining historical overlap relief had to be deducted during this transitional step.

If this calculation created an artificially inflated tax bill (known as "transition profits"), HMRC did not tax it all at once. Instead, these transition profits were automatically spread evenly over five tax years:

1. 2023/24 Tax Year (First 20% instalment)
2. 2024/25 Tax Year (Second 20% instalment)
3. 2025/26 Tax Year (Third 20% instalment)
4. 2026/27 Tax Year (Fourth 20% instalment — Current Year)
5. 2027/28 Tax Year (Final 20% instalment)

When preparing your 2026/27 Self Assessment return, you must ensure that your mandatory 20% transition profit instalment is added to your standard trading profits for the year. If your business has ceased trading during this period, any remaining untaxed transition profits are brought forward and taxed in full immediately.

Key Points to Remember

• All unincorporated businesses are taxed strictly on profits arising between 6 April and 5 April.
• If your year-end is not aligned with April, you must mathematically apportion your profits from two separate sets of accounts.
• Overlap profit and overlap relief have been permanently abolished.
• For the 2026/27 tax return, you must include your fourth 20% instalment of deferred 2023/24 transition profits.
• If you rely on provisional estimates due to accounting date mismatches, you must amend your return once your actual figures are finalized.