Reporting CGT past losses. Before self-assessment or during?

As I understand it, you can claim past losses up to 4 years after the end of the tax year that you disposed of the asset. so for example: - 24/25 disposed of asset with a **-£1000** loss - 25/26 disposed of asset with a **+£10000** profit My understanding is that the losses made in previous tax years carry forward and mean that you only pay CGT on the overall profit. i.e.: £10000 - £1000 = **£9000** (and then you deduct from that the tax-free CGT allowance of £3000, so actually only £6000 is taxable) My question is, should one report the losses in advance of filing self-assessment for the current year? i.e., report the losses via a letter to HMRC, then later file the self-assessment at the end of 25/26 declaring the £10000 profit (with no reference to previous losses), and then HMRC will see the past losses in your records unprompted, and bill you accordingly? Do you declare/reference the previous years losses within self-assessment for the year in which you want to make use of them? Second question: is there a template for the letter for declaring past losses? I'm not sure how much detail is expected. Thanks!

Not HMRC, if you are not SA for 24/25 send HMRC a letter with the calculation of the loss. Then when you complete SA for 25/26 you show the gain for 25/26 and put the loss figure in loss b/fwd box on the CGT pages to the return and the SA calculation will take the loss off the gain and allow the exemption and calculate tax on the balance. No template Im aware of.

I realise now my example doesn't really show my issue properly as I could still file self-assessment for the £1000 loss in 24/25. Instead imagine that occurred in 23/24 where self assessment period has since closed.

Hi Chris,
Thank you for your two posts.
Yes you need to report them first and provide full details - 
•  a declaration to the effect that all the particulars given in the claim are correctly stated to the best of the information and belief of the person making the claim,
•  details of the asset disposed of
•  quantification of the loss – this is most easily done with a computation showing the date of disposal, the amount of proceeds received, the allowable expenditure and the loss that results from the disposal.
•  signed by the individual making the claim. This will be acknowledged to then allow you to claim losses in the tax year that they are being used.
Thank you.