Basis Period Reform Case Study: 2026/27 Transition Profit

The Basis Period Reform is the most significant change to the taxation of unincorporated businesses in a generation. From the 2024/25 tax year, all self-employed individuals and partners will be taxed on profits arising in the tax year (6 April to 5 April), regardless of their accounting date. This new ‘tax year basis’ replaces the old, complex ‘current year basis’ rules.

While the reform’s objective is to create a simpler and fairer system, the 2023/24 tax year acts as a crucial transition period that presents unique challenges.

This case study illustrates how a typical partnership navigates the transitional rules, uses its historic overlap relief, and prepares for the simpler system ahead.

The Client: A Partnership Facing Change

Let’s meet “DesignCo”, a successful creative design partnership.

• Business: A partnership with two partners.
• Accounting Date: They prepare their annual accounts to 30 September each year.
• The Problem: Because their accounting date is not 31 March or 5 April, they are directly affected by the Basis Period Reform. They have unused ‘overlap profits’ from when they started trading, but the partners are unsure what this means or how the transition will affect their tax bill.

The old system created these ‘overlap profits’ because, in the early years of trading, certain periods of profit were taxed twice. Overlap relief was created to compensate for this, but it could only be used when the business ceased or changed its accounting date. The Basis Period Reform changes all of this.

The Solution: Navigating the 2023/24 Transitional Year

The 2023/24 tax return is the key to managing the transition. For DesignCo, this means calculating the profits for a basis period that is longer than 12 months.

Step 1: Calculating the Transitional Basis Period

The basis period for 2023/24 is made of two parts:

1. The Standard Part: The usual 12 months following the end of the previous basis period. For DesignCo, this is 1 October 2022 to 30 September 2023.
2. The Transition Part: The period immediately after the standard part, running to the end of the tax year. For DesignCo, this is 1 October 2023 to 5 April 2024.

This gives DesignCo a total basis period for 2023/24 of 18 months and 5 days.

Step 2: Calculating Profits and Deducting Overlap Relief

DesignCo must now calculate its profits for this extended period and, crucially, use all its brought-forward overlap relief. This is the final opportunity to do so.

Step 3: Identifying and Spreading the Transition Profits

The reform recognises that taxing over 18 months of profit in one year would create a significant cash flow burden. Therefore, the additional profit arising from the ‘transition part’ is automatically spread over five years.

• Total Taxable Profit: £120,000
• Less: Standard Part Profit: (£90,000)
• Equals: Transition Profit: £30,000

This £30,000 transition profit is spread. For the 2023/24 tax year, DesignCo’s final taxable profit is:

• Standard Profit (£90,000) + One-fifth of Transition Profit (£6,000) = £96,000

The remaining £24,000 of transition profit will be taxed in equal instalments of £6,000 over the next four tax years (2024/25 to 2027/28).

The Result: A Clear Path Forward

By following this process, DesignCo has successfully navigated the transition.

• Compliance: They have correctly calculated their profits for the 2023/24 transitional year.
• Tax Efficiency: They have fully utilised their £18,000 of overlap relief, which might otherwise have been lost or forgotten.
• Cash Flow Management: The five-year spread of the transition profits makes their tax liability manageable.
• Future Simplicity: From the 2024/25 tax year onwards, their tax calculation is simplified. Their taxable profit will be the profit arising between 6 April and 5 April each year. This provides a clearer link between profits earned and tax paid and removes complex basis period rules for good.

Key Takeaways for Unincorporated Businesses

• Identify if You Are Affected: If your business does not have a 31 March or 5 April accounting date, you must manage this transition.
• Find Your Overlap Relief Figure: This is essential. The 2023/24 tax year is the last chance to use it.
• Plan for the Transition Profits: Understand how the five-year spread impacts your tax liability and financial planning.
• Consider Your Accounting Date: While not mandatory, changing your accounting date to 31 March or 5 April will simplify the need for apportionment in the future.

For detailed legislative guidance, professionals should refer to Schedule 1 of the Finance Act 2022 and HMRC’s Business Income Manual, specifically the sections from BIM81200 onwards.