The UK rules, section 104, same-day, and the 30-day rule

This page is a stub. The full guide is written against the shipped app version and is on its way.

Every UK crypto disposal answers one question, which purchase is this sale matched against. The answer decides your cost basis, so it decides your gain, and UK law answers it with three rules applied in strict order.

First, same-day. Anything you bought on the same day as the sale matches first, section 105 TCGA 1992.

Second, the next 30 days. Anything you buy back within the 30 days after the sale matches next, newest rules of the game for anyone selling to realise a loss and rebuying, section 106A. The boundary is precise, the first clear day is day 31, sell on 13 August and a rebuy is only outside the window from 13 September. The full worked example is in the bed and breakfast rule post.

Last, the section 104 pool. Everything else lives in one pool per asset at average cost, section 104, the rule doing the everyday work, explained at length in the section 104 pool post.

Two things traders miss. A crypto-to-crypto swap is a disposal of one asset and an acquisition of the other, both sides count, which is how active traders trigger the 30-day rule weekly without noticing. And the rules apply per asset, your ETH pool and your BTC pool never touch.

Every disposal in your reports names which of these rules matched it, that is reading citations, and each rule’s implementation is tested against HMRC’s own worked examples, published on the verification page.