If you sell assets like rental properties, shares or cryptocurrency at a profit, you have to consider whether you have Capital Gains Tax (CGT) to pay for the tax year. But you might also have sold some other assets at a loss in the year - if so, you can deduct those losses from your profits to get to a net profit or loss for the year.
If you wind up with net gains, but they’re less than the Annual Exemption - this year, £12,300 - then there’s no CGT to pay and you move on.
If you wind up with net losses, then you get to hang on to them. If, in a future year, you have net gains over the Annual Exemption, then you can set those old losses against them, using just enough to get down to the Annual Exemption and eliminate any tax that would otherwise be due, and carrying anything left forward still further. It might be that you never have sufficient gains in a future year to ever use them, in full or at all, but they’re there if you do.
Capital losses are about to become more valuable, because the Annual Exemption is being cut sharply. Next year, it’ll be only £6,000, the year after, £3,000. That means that more taxpayers will be dragged into the CGT net.
There’s an easy trap to fall into. In order to establish entitlement to most losses, you generally have to claim them within four years of the tax year they occur in (there’s a shorter deadline for some niche cases). That could be on your Self Assessment Tax Return, if you have to submit one for the year, or you could just do it by letter or some other way. But you have to somehow tell HMRC before that deadline. We recently saw a case where someone had losses in 2013-14, and gains over the Annual Exemption in 2021-22. Unfortunately, HMRC hadn’t been told about the losses, so it was too late for the taxpayer to take advantage of them. Fortunately, the losses weren’t enormous.
A final consideration - you might need to make a judgement about how much effort you’re prepared to put into quantifying losses that you’re not certain will be useful. If someone is pretty sure that they’ve got losses this year, but they’ll have to put research time into establishing the history of the share transactions, or pay an adviser to work out the actual loss, should they do it when they don’t anticipate having an opportunity to use the loss in future? To begin with they can kick the can down the road, by making a note for three years’ time to look at it again. The point will come, though, where they have to make a judgement on whether to do the work before the deadline passes.