Tips & Advice
Self Assessment CGT Adjustment: How To Correct Your 2024/25 Return
Published: 10 September 2025
Reading time: ~6 minutes
Why Some 2024/25 Tax Returns May Now Be Wrong
The government increased CGT rates on 30 October 2024, and it seems HMRC’s Self Assessment system was not configured to apply the correct rate to post-October disposals. Many taxpayers who sold chargeable assets after that date and filed early may therefore have paid too little CGT without realising. A Self Assessment CGT adjustment may now be required to put things right.
Why Some 2024/25 Returns Are Affected
From 30 October 2024, the Government increased the CGT rates applying to assets other than residential property and carried interest. The changes were:
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Basic rate taxpayers: from 10% to 18%
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Higher rate taxpayers: from 20% to 24%
However, HMRC’s online Self Assessment software and some commercial packages did not initially handle gains that straddled the rate-change date. In other words, if you reported a disposal after 30 October 2024, the system might have applied the old lower rates or failed to split the gain correctly.
HMRC has confirmed on GOV.UK that users of both its own service and commercial software should check that the correct rates were applied.
HMRC’s Response And New Calculator
To address the issue, HMRC released a CGT adjustment calculator. This tool helps taxpayers manually recalculate their total CGT liability if a disposal occurred on or after 30 October 2024.
HMRC is now writing to individuals who:
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Made a post-October disposal but did not include an adjustment, or
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Made an adjustment that appears to be wrong
Each letter asks the taxpayer to check their figures using the HMRC CGT calculator and, within 30 days, either amend the return or confirm it is correct.
What You Should Do
If you reported any disposals of shares, investments, or other non-property assets after 30 October 2024, take these steps:
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Gather details of each disposal, including dates, sale proceeds and allowable costs.
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Use the HMRC CGT calculator to see whether extra tax is due.
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Amend your Self Assessment online if the tool shows a higher liability.
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If your figures are correct, respond to HMRC as directed in their letter.
HMRC will charge interest on any underpaid CGT from the original due date, so make sure you pay the correct amount on time.
Practical Checks For Tax Software Users
If you used commercial software to submit your return, double-check whether it applied the correct CGT rates for disposals after 30 October 2024. HMRC’s own notice states: “If you submit your Self Assessment tax return using commercial software you need to check the correct rates are used.”
Where a mismatch is found, file an amendment through HMRC or your software provider. A Self Assessment tax return review by a professional can save time and prevent costly errors.
How Can SCCS Accountants Help?
At SCCS Accountants, we support clients UK-wide with Capital Gains Tax planning and compliance. We can:
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Check your CGT computation working sheet and adjustment figures
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Liaise with HMRC to clarify any queries or interest charges
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Advise on allowances and timing to manage future disposals
For property investors, our property tax services ensure that gains, reliefs and deadlines are all correctly handled.
FAQs on Self Assessment CGT Adjustment
When do you need to make a CGT adjustment?
You need to make a CGT adjustment if you disposed of chargeable assets after 30 October 2024 and filed a 2024/25 return before HMRC’s software applied the new rates. HMRC letters highlight affected cases.
How does the HMRC CGT calculator help?
The calculator applies the correct CGT rates before and after 30 October 2024, showing any extra tax due. It guides you on how to amend your Self Assessment if needed.
What are the new Capital Gains Tax rates?
From 30 October 2024, the rate is 18% for basic rate taxpayers and 24% for higher rate taxpayers on most non-property disposals. Earlier disposals remain at 10% and 20%.
How to calculate CGT on shares bought at different times?
UK share pooling and same-day rules apply. You need to match disposals to acquisitions using HMRC’s method, which the CGT calculator can accommodate.
How can I reduce future CGT liabilities?
Plan ahead by using your annual Capital Gains Tax allowance and considering the timing of disposals. Professional advice is valuable where multiple assets or reliefs are involved.
Disclaimer
This article is provided for general informational purposes only and does not constitute tax, legal, accounting, or financial advice. The information is based on UK law and HMRC guidance as at the date of publication, but rules and interpretations may change. We do not accept any liability for actions taken, or not taken, based on this content. Always seek tailored advice from SCCS Accountants before making financial or business decisions.