Tax Year Basis Shift 2026: What Unincorporated Businesses Need to Know

For years, many sole traders and partnerships in the UK have operated under a set of tax rules that allowed them to choose their own "accounting year-end." You might have chosen 31 December to match the calendar year, or perhaps 31 July to give yourself more time after the busy summer season. However, HMRC is in the midst of a massive shake-up known as the Basis Period Reform.

By the time we reach the 2026/27 tax year, the "Current Year Basis" will be a thing of the past. All unincorporated businesses: meaning sole traders, partnerships, and LLPs: will be taxed on the profits they earn during the actual tax year (6 April to 5 April), regardless of when their own accounting year ends.

If you haven’t already prepared for this change, 2026 is the year where the "new normal" fully settles in. In this guide, we’ll break down what this shift means for you, how to handle the transitional hurdles, and why your choice of accountant matters now more than ever.

What is the Tax Year Basis?

Up until recently, if your business year ended on 31 December 2023, those profits were taxed in the 2023/24 tax year. This was known as the "current year basis." It was flexible, but it created a disconnect between the business's books and the government's tax calendar.

From the 2024/25 tax year onwards, the UK moved to the Tax Year Basis. This means for the 2026/27 tax year, you are taxed on the profits arising specifically between 6 April 2026 and 5 April 2027.

Why is this happening?

HMRC’s primary goal is alignment. By moving everyone to the same schedule, they are preparing for the rollout of Making Tax Digital (MTD) for Income Tax Self-Assessment (ITSA). When tax reporting becomes digital and quarterly, having everyone on the same calendar makes the system much easier for HMRC to manage: even if it creates a bit of a headache for business owners in the short term.

The 2026/27 Landscape: Apportionment and Accuracy

If your accounting year-end is already 31 March or 5 April, congratulations! You are largely unaffected. HMRC treats 31 March as the same as 5 April for these purposes, so your accounts and your tax return will continue to match up perfectly.

However, if your year-end is anything else (let’s say 30 September), 2026 brings a unique challenge called apportionment. Because you are being taxed on the profits from 6 April 2026 to 5 April 2027, you will have to "blend" figures from two different sets of accounts:

1. A portion of the profits from your year ending 30 September 2026.

2. A portion of the profits from your year ending 30 September 2027.

This requires careful calculation. You can choose to apportion profits based on days or months, provided you are consistent. Many small businesses find this confusing, which is why finding an expert accountant is often the best way to avoid expensive mistakes. If you are comparing your options, Accounting Services UK: The Complete Guide to Finding the Right Accountant for Your Business can help you narrow down the right fit.

Dealing with Transition Profits and Spreading

The shift to the tax year basis didn’t happen overnight. Most businesses went through a "transitional year" in 2023/24. During that time, many ended up with a "long" basis period: sometimes taxing 18 months of profit in a single year to get the timeline aligned.

To prevent businesses from being hit with a massive, one-off tax bill, HMRC introduced Transition Profit Spreading. This allows you to spread the "extra" profit from that transition over five tax years.

By the 2026/27 tax year, you will likely be in Year 4 of that spreading period. This means:

• You still have a slice of "transition profit" to add to your 2026/27 tax return.

• You need to keep track of how much is left for the final year (2027/28).

• If your business ceases trading in 2026, all remaining transition profits become taxable immediately.

Keeping track of these "spread" profits is vital for your cash flow. If you forget to account for them, you might find yourself with a higher-than-expected bill come January. If you're feeling overwhelmed, checking our small business accounting resources can help you stay on top of these deadlines.

What Happened to Overlap Relief?

In the old system, when businesses first started or changed their year-ends, they often paid tax on the same profits twice. These were called overlap profits.

HMRC mandated that all remaining Overlap Relief must have been used up during the 2023/24 transitional year. If you didn't claim it then, you may have missed your chance. However, if you are still dealing with late filings or complex partnership adjustments, it is worth checking with a professional to ensure no relief was left on the table. Overlap relief was the "antidote" to double taxation, and making sure it was correctly applied to your transition profits is a key part of tax preparation.

Specifics for Partnerships

For partnerships and LLPs, the basis period reform is handled at the partner level, not the partnership level. This is a crucial distinction.

• The partnership itself can still prepare accounts to its traditional year-end (e.g., 30 June).

• Individual partners, however, must report their share of those profits on their personal tax returns using the tax year basis (6 April to 5 April).

This can lead to a scenario where different partners have different "transition profit" amounts based on when they joined the firm. It makes the partnership’s tax administration significantly more complex, especially for large firms with many partners.

Practical Steps for 2026

As we move through 2026, there are several steps sole traders and partners should take to ensure they stay compliant and efficient:

1. Consider Changing Your Year-End: Many businesses are choosing to officially change their accounting year-end to 31 March or 5 April. This removes the need for annual apportionment and makes life much simpler. You don't usually need to ask HMRC for permission; you simply change the date on your next set of accounts.

2. Review Cash Flow: With transition profits still being added to your tax bills until 2028, your tax liability might be slightly higher than your actual trading profits suggest. Ensure you are setting aside enough for your Self-Assessment payments.

3. Audit Your Records: Ensure you have clear working papers showing how you calculated your apportioned profits. HMRC expects a "reasonable and consistent" approach.

4. Prepare for MTD: The Tax Year Basis shift is the foundation for Making Tax Digital. Use 2026 to transition to online accounting software if you haven't already. Our guide on Transitioning to MTD: A Step-by-Step Guide for Sole Traders explains what to expect and how to get ready.

How Accountant Search Can Help

The 2026 tax landscape is more rigid than it used to be. While the Tax Year Basis aims for simplicity in the long run, the current transitional period is anything but simple. Managing apportionment, spreading transition profits, and preparing for the digital tax revolution requires expert guidance.

At Accountant Search, we specialize in matching SME businesses with the perfect accounting partners. Whether you are a sole trader looking to align your year-end or a partnership needing to navigate complex partner-level reporting, we can connect you with local experts who understand the nuances of the 2026 tax year.

Don't wait until the January deadline to realize your tax calculations are out of sync. Stay ahead of the curve and ensure your business is ready for the future of UK tax. If you are not sure where to start, read How to Find an Accountant in the UK: A Step-by-Step Guide for Business Owners.

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Author: Jessica