Corporation Tax is one of those bills that feels straightforward until you actually run the numbers. Most directors assume there are two rates, 19% or 25%, and their company falls into one or the other. The reality for a lot of small businesses is messier, and getting it wrong means either overpaying or under-budgeting for a bill that lands nine months later.
For 2026, the structure introduced back in April 2023 still applies, with no rate changes announced. But a couple of the surrounding rules have shifted, and the middle band still trips people up. Here is how UK Corporation Tax works in 2026 and where the real planning opportunities sit.
The 2026 Corporation Tax Rates
There are two headline rates, with a sliding scale in between:
- 19% small profits rate on taxable profits up to £50,000
- 25% main rate on taxable profits above £250,000
- Marginal relief on profits between £50,000 and £250,000, giving an effective rate that climbs gradually from 19% towards 25%
The 19% rate covers most owner-managed companies with modest profits. If your company makes £40,000 taxable profit for the year, you pay 19%, which comes to £7,600. No marginal relief calculation needed at that level.
Above £250,000, the full 25% applies with no relief. It is the band in between where things get interesting, and where most growing small businesses actually sit.
How Marginal Relief Really Works
Here is the part that catches directors out. Within the £50,000 to £250,000 band, the calculation does not apply a flat 25%. Instead, your company is charged at 25%, then marginal relief is deducted to bring the effective rate down.
The formula uses a fraction of 3/200, but your CT600 software does the maths, so you never work it out by hand. What matters is understanding the effect: for a company with £90,000 of taxable profit and no associated companies, the effective rate works out at around 22.3%, not 25%.
There is a sting, though. Every extra pound of profit inside that band is actually taxed at an effective marginal rate of 26.5%, higher than the main rate itself, because the relief is being withdrawn as profits rise. So a company sitting just inside the band pays a surprisingly high rate on its top slice of profit. That single fact drives most of the planning worth doing.
The Associated Company Trap
This is the rule that quietly costs directors the most. The £50,000 and £250,000 thresholds are not fixed. They get divided between "associated companies" under common control.
If you run one company, you get the full thresholds. Add one associated company and they halve, to £25,000 and £125,000. Add two, and they drop to roughly £16,667 and £83,333 each. Suddenly a company you assumed was safely on the 19% rate is paying marginal rates instead, purely because you also control another company.
Plenty of directors set up a second company for a new venture, a property holding, or a side project without realising it drags both companies into higher effective rates. If you hold multiple companies, this is exactly the kind of structure worth reviewing properly rather than assuming each one stands alone. Aspire's business advisory and tax support covers this sort of structure review, because the fix is often simpler than the tax it saves.
Capital Allowances Changed in April 2026
One genuine change for 2026: following the Autumn Budget 2025, the main writing-down allowance dropped to 14% from April 2026, down from 18%. Special rate pool assets stay at 6%.
In plain terms, tax relief on plant and machinery in the main pool now comes through more slowly. This makes the timing of larger purchases more important than it used to be, and it makes full expensing, where it is still available, more valuable while it lasts. If your company is planning a significant equipment or asset purchase, the timing around your year-end can materially change when you get the relief.
Smart Planning Tips for 2026
A few practical moves make a real difference to the final bill.
Watch the £50,000 line. Because of that 26.5% marginal rate, a company sitting just above £50,000 pays a heavy rate on the profit above it. Legitimately managing the timing of income and allowable expenses around your year-end can keep more profit in the 19% band. This is not about hiding profit, it is about when costs and income fall.
Time your asset purchases. With writing-down allowances now lower, bringing a planned purchase forward or back relative to your accounting period end affects which year the relief lands in. Buy at the wrong point and you wait an extra year for relief you could have had sooner.
Claim every allowable expense properly. The simplest way to reduce taxable profit is to make sure genuine business costs are recorded and claimed. Missed expenses mean tax paid on profit that was never really there. This depends entirely on clean records throughout the year, not a scramble at year-end.
Review associated companies before year-end. If you control more than one company, check how the shared thresholds affect each one. Sometimes the structure can be adjusted; sometimes it just needs to be planned around. Either way, finding out in advance beats finding out on the tax bill.
Keep proper records for relief claims. HMRC has increased its focus on small company accuracy and is reviewing marginal relief claims more closely. Weak records make any claim harder to defend if questioned.
Don't Forget the Deadlines
Corporation Tax has an unusual quirk: the payment deadline comes before the filing deadline. For most small companies, the tax is due 9 months and one day after your accounting period ends, while the Company Tax Return itself is not due until 12 months after.
That means you often have to pay before you file. Directors who wait until the filing deadline to think about the numbers can find the payment was already overdue. Planning the cash for Corporation Tax well ahead of the payment date is as important as calculating it correctly.
Getting the Numbers Right
Corporation Tax in 2026 is not just about picking 19% or 25%. It is about knowing your true taxable profit, checking whether associated companies shrink your thresholds, timing purchases around the lower capital allowances, and planning the cash before the payment date arrives.
Getting any of those wrong can turn a sensible forecast into a nasty surprise. If your profits are near the thresholds, you run more than one company, or you just want your Corporation Tax planned properly rather than estimated, speak to ASPIRE UK TAX ACCOUNTANTS before your year-end, not after.
ASPIRE UK TAX ACCOUNTANTS is an ACCA registered UK practice helping limited companies, startups and SMEs with Corporation Tax, year-end accounts, tax planning, bookkeeping, VAT and HMRC support.
