Basis period reform landed in the 2023/24 tax year and, for most sole traders and partnerships, got treated as a one-off administrative shift and then forgotten about. For a property trading business, that's a mistake. The reform didn't just change how one year was taxed. It created a slice of extra profit that, for many businesses, is still being taxed in instalments today, three tax years after the change actually happened.
Who this actually catches
Basis period reform only applies to unincorporated trading businesses, meaning sole traders and partnerships that buy, build and sell property as trading stock rather than hold it for rental income. That distinction matters immediately, because it's the same line drawn in our guide to trading versus investment: a landlord with a letting business was already taxed on a tax-year basis before the reform, since property income has always run 6 April to 5 April. Companies aren't affected either, since corporation tax has its own accounting period rules untouched by this change. The reform is squarely aimed at property developers and small building businesses operating as sole traders or in partnership, particularly ones that historically drew up accounts to a date other than 5 April, commonly for practical reasons like aligning with a site's completion schedule rather than the tax calendar.
What actually happened in the transition year
Before the reform, a trading business with, say, a 30 June accounting date was taxed in a tax year on the profits of the accounting period ending within it, which meant profits earned as late as June 2023 could still be taxed as part of the 2023/24 return. From 2023/24 onwards, every unincorporated trading business is instead taxed on the profit actually arising in the tax year itself, 6 April to 5 April, regardless of its accounting date.
Getting from the old basis to the new one for a business with a non-tax-year accounting date meant taxing an extra stub period, the months between the old accounting date and the following 5 April, on top of the usual twelve months, in the 2023/24 return alone. That additional slice is the transition profit, and it was reduced by any unused overlap relief the business had accumulated, before being spread, by default, evenly across five tax years from 2023/24 to 2027/28 rather than taxed in full in one go.
Overlap relief finally did its job, for better or worse
Overlap relief exists because the old basis period rules could tax the same early profits twice, once as part of the first accounting period and again as part of the second, and businesses carried that double-taxed amount forward as relief to use whenever their accounting date changed. Plenty of long-established property trading businesses that never changed accounting date simply carried overlap relief on their books for years, sometimes decades, without ever having an occasion to use it. Basis period reform was that occasion, applied automatically to reduce the transition profit calculated in 2023/24.
Where the figure had been tracked properly since the business started, this was a clean, welcome offset. Where records had been lost over years of changing accountants, software or ownership, some businesses found themselves with a transition profit calculation they couldn't fully substantiate a deduction against, turning what should have been routine relief into an argument about evidence going back further than anyone had kept clean records for.
The trap that's still running now
This is the part that catches people out well after the reform itself is old news. Unless a business chose to accelerate some or all of its transition profit into an earlier year, typically to use up losses or reliefs available at the time, the default spreading means a fifth of the original 2023/24 transition profit lands on each tax return from 2023/24 through to 2027/28. A developer preparing their 2026/27 return now, three years after the change, can still find an extra slice of profit due purely because of a calculation made back in 2023/24 that nobody in the business has thought about since.
That's a genuine cash flow issue for a trading business where profit is lumpy and tied to when sites actually complete, which is most of them. A year with a quiet site programme and modest trading profit can still carry an unexpected addition from an old transition calculation, pushing the tax bill higher than the year's actual trading would suggest, in a way that's easy to miss if the spreading schedule isn't tracked separately from ordinary annual profit.
Where incorporation changes the picture
A property trading business that incorporates part way through the spreading period doesn't escape the remaining transition profit; any outstanding balance is generally accelerated and brought into charge on cessation of the trade rather than carried into the company. That's worth factoring into the timing of a decision to move a trading business into a company structure, alongside the wider comparison we cover when developers ask when an SPV actually pays off, since the remaining transition profit is a real, quantifiable cost of the timing of that move, not just an abstract consideration.
It also interacts with how losses are used. A business carrying forward trading losses, of the kind discussed in our guide to sideways loss relief, may find those losses are exactly what makes accelerating remaining transition profit into a particular year worthwhile, rather than leaving it to land in five equal, uncontrolled instalments regardless of what else is happening in the business that year.
Common mistakes
- Assuming basis period reform was a 2023/24 event only, with nothing left to think about in later years
- Forgetting the outstanding spread of transition profit when budgeting for tax on a quieter trading year
- Not checking historic overlap relief records before the transition calculation is finalised, and losing relief that should have reduced it
- Incorporating a trading business without checking what happens to the remaining transition profit balance on cessation
- Treating landlords and property trading businesses as subject to the same rules, when only the latter were ever affected
What this means for property trading businesses
If your business changed accounting date away from 5 April at any point before 2023/24 and traded as a sole trader or partnership, check now whether there's still transition profit due on returns for 2025/26 through 2027/28, rather than discovering it as a surprise addition when the return is prepared. It's worth reviewing whether accelerating the remaining balance into a lower-profit year, or against losses available now, would beat leaving it on the default five-year schedule. This is exactly the kind of structural detail that gets missed once the headline reform has faded from the news, and it sits alongside the broader planning conversation we have with trading property businesses about structuring profit and the right vehicle to hold a development pipeline in going forward.
Common questions
Does basis period reform affect landlords as well as property developers?
No. Basis period reform only affects unincorporated trading businesses, meaning sole traders and partnerships that buy, build and sell property as trading stock. Property income from letting was already assessed on a tax-year basis before the reform, so landlords with a rental business, as opposed to a trade, saw no change to how their profits are taxed.
What happened to my unused overlap relief from when I started trading?
Any overlap relief built up in the early years of trading, from profits taxed twice under the old basis period rules, was set against the transition profit calculated for the 2023/24 tax year. For businesses that had traded for many years without changing accounting date, this was often the first time that relief had ever been used.
Can I still be paying tax on transition profit in 2026/27?
Yes, if the transition profit calculated for 2023/24 wasn't accelerated at the time. Unless a business chose to bring forward more of it earlier, the default spreads the remaining transition profit evenly across the five tax years from 2023/24 to 2027/28, so a slice of it is still due on returns being prepared now.
Kieran Holsgrove is a Director and Co-Founder of Grafene Accounting, the property tax specialist firm based in Liverpool. He advises property developers, investors and landlords across Merseyside, Greater Manchester, Lancashire and Cheshire on tax structuring, developer VAT, SDLT and the long-view decisions that compound over the life of a portfolio.
This article is general information, not personal tax advice, and tax rules change. Your own position depends on facts we cannot see from here — please take advice before acting on anything above.