Underpaid CGT for 2024/25? The box 51 fix before 31 Jan 2027
If you sold shares or funds after 30 October 2024 and have already filed your 2024/25 return, there's a decent chance the tax it worked out is too low. Not because you got anything wrong — because HMRC's own Self Assessment calculation doesn't apply the new capital gains rates for that year. You have to add the difference yourself, in box 51.
Here's what's going on and how to fix it while you still can.
Why the return under-calculates
The Autumn Budget 2024 raised the main CGT rates on 30 October 2024, part-way through the tax year: from 10% to 18% for basic rate gains, and from 20% to 24% for higher rate gains. Shares, funds, and other non-property assets all moved.
The problem is timing. The rate change landed mid-year, but the Self Assessment tax calculator was built around the old rates. So for 2024/25 it works your gains out at the old 10%/20% rates and stops there. HMRC's own guidance says it plainly: the return "will not automatically calculate at the new main rates."
The gap doesn't fix itself. HMRC expects you to work out the extra tax on your post-30-October gains and enter it as an adjustment. The SA108 notes are explicit that box 51 "may be required to increase the tax due because of the increase in certain Capital Gains Tax rates for gains on disposals on or after 30 October 2024."
A worked example
Say you're a higher rate taxpayer. During 2024/25 you made an £8,000 gain selling shares in August and a £5,000 gain selling a tracker fund in November. Total gains £13,000, minus the £3,000 annual exempt amount, leaves £10,000 taxable. You set the allowance against the higher-taxed November gain, because that saves the most.
| Disposal period | Taxable gain | Correct rate | Tax |
|---|---|---|---|
| 6 Apr – 29 Oct 2024 | £8,000 | 20% | £1,600 |
| 30 Oct 2024 – 5 Apr 2025 | £2,000 | 24% | £480 |
The correct tax is £1,600 + £480 = £2,080.
Left to itself, the return charges the old 20% rate across the board: £10,000 × 20% = £2,000. That's £80 short. The £80 is the extra 4% on the £2,000 of November gains, and it's exactly what goes in box 51. Miss it, and HMRC's calculation says you owe £2,000 when you actually owe £2,080.
The shortfall is bigger for basic rate gains, where the rate jumped 8 points (10% to 18%) rather than 4. And it gets fiddlier when a gain straddles the basic and higher rate bands, which is the main reason HMRC built a tool to do the sum rather than leaving it to a mental estimate.
How to work out box 51
Use HMRC's adjustment calculator. It asks for your gains before and after 30 October, your losses, and your allowance, then returns the figure to enter. That figure goes in box 51, "Adjustments to Capital Gains Tax," on the SA108. It's normally a positive number — extra tax. In the rare case the adjustment reduces your tax, you put a minus sign in front of it.
The one input the calculator can't guess is which gains fall on which side of 30 October. That's down to the date you sold, not the date you bought — even shares held in a Section 104 pool for years are dated by the disposal. Our CGT calculator already splits your 2024/25 disposals at 30 October and shows the gain in each period, so you can read the two numbers straight off the report and drop them into HMRC's tool.
You have until 31 January 2027
The deadline for the 2024/25 return was 31 January 2026, but you can amend a return for 12 months after that — so until 31 January 2027. If you filed online, sign in, open the 2024/25 return, and change box 51. If you used commercial software, amend through it. After the window closes you'd have to write to HMRC instead, which is slower.
It cuts both ways. If all your 2024/25 disposals happened before 30 October, or your software charged 24% on gains that should have been taxed at the old rates, you may have overpaid — the same box 51, with a minus sign, gets it back.
The short version
For 2024/25 only, the Self Assessment return doesn't price in the 30 October rate rise. If you sold after that date, work out the difference with HMRC's calculator and put it in box 51 before 31 January 2027. From 2025/26 the rates are unified all year and this quirk goes away — for the background on the change itself, see CGT rate changes in 2024/25.