The same-day and 30-day (bed and breakfast) rules
Before HMRC touches your Section 104 pool, it applies two matching rules. They exist to stop people manufacturing losses by selling and quickly rebuying — and they change which cost basis your disposal uses.
The matching order
Every disposal is matched against acquisitions in this strict order:
- Same-day rule — acquisitions of the same asset on the same day as the disposal (TCGA 1992 s.105).
- 30-day rule — acquisitions in the 30 days after the disposal (TCGA 1992 s.106A).
- Section 104 pool — everything else, at the weighted-average cost.
Why “bed and breakfasting” no longer works
Investors once sold an asset to crystallise a loss, then bought it back the next morning to keep their position — “bed and breakfasting”. The 30-day rule closed this: if you rebuy within 30 days, your disposal is matched against that repurchase, not the cheap historic pool, so the loss largely disappears.
This catches a lot of crypto traders by surprise. Selling at a loss and rebuying the same coin a week later usually does not give you the loss you expected — the cost basis resets to the repurchase price.
Planning around it
If you are harvesting a loss before the tax year ends, the 30-day rule means you cannot immediately rebuy the identical asset. Maneta’s simulator warns you before a disposal if a purchase in the last 30 days would trigger the rule, so you are not surprised at filing time.