Bed and Breakfasting Shares: The 30-Day Rule Explained (2026)

What the 30-Day Rule Actually Does

Capital Gains Tax on shares is calculated based on the difference between what you paid (the "base cost") and what you sold for. If you're sitting on a loss, selling crystallises that loss, which can then be used to offset gains elsewhere and reduce your CGT bill — a legitimate and common strategy known as tax-loss harvesting.

The problem HMRC identified decades ago: nothing stopped an investor selling shares purely to bank the loss (or bank a gain to use up the annual CGT exempt amount), then immediately buying the identical shares straight back — meaning your actual investment position barely changed, but your tax position improved.

The 30-day rule (formally part of the share identification/matching rules) closes this by dictating the order in which a sale is matched against your share purchases:

1. Same-day rule — a sale is first matched against any purchases of the same share made on the same day.
2. 30-day rule — if not fully matched, the sale is next matched against purchases made in the following 30 days (not preceding — this is a forward-looking rule).
3. Section 104 pool — only after those two rules are exhausted is the disposal matched against your longer-held "pooled" holding (the average cost of all your older shares of that class).

If you sell 1,000 shares today and buy 1,000 of the same shares back within the next 30 days, your "sale" is matched against the shares you just rebought — not your original pool — which usually results in a negligible gain or loss, because the sale price and the rebuy price are close together in time.

---

Worked Example

Suppose you hold 5,000 shares in Company X, bought years ago at an average cost of £2.00/share (pooled cost £10,000). The shares now trade at £1.20, so you're sitting on an unrealised loss of £4,000 if you sold at market price.

Attempted bed and breakfasting (blocked):

Because the repurchase falls within 30 days, the disposal is matched against the Day 5 purchase, not the original £10,000 pooled cost. The "loss" for CGT purposes becomes based on £6,000 proceeds vs £6,250 repurchase-matched cost — effectively a small loss of £250, not the £4,000 loss you were trying to crystallise against your original holding.

Legitimate alternative — Bed and ISA:

Because the repurchase happens inside a different account type (an ISA), it is not treated as the same disposal-and-reacquisition sequence for share matching purposes in the way a same-account repurchase would be — the loss on the original GIA holding stands, and the shares are now sheltered from CGT going forward inside the ISA.

---

Bed and ISA vs Bed and SIPP

Both routes are widely used by investment platforms, which often offer a "Bed and ISA" service each tax year specifically to help investors move general account holdings into their annual ISA allowance while banking any available loss (or using up the annual CGT exempt amount efficiently on a gain).

---

Practical Traps to Avoid

1. Don't assume 30 calendar days is the same as 30 trading days — the rule uses calendar days.
2. Remember the rule works forward only — buying the same shares in the 30 days before a sale doesn't trigger the same-day/30-day matching in the same way; the mechanism specifically targets repurchases after disposal.
3. Fund switches and share reclassifications can sometimes count as a "disposal and reacquisition" for CGT purposes even without you actively trading — check with your platform if a fund merger or share class change affects your matching position.
4. Different share classes or listings of the same company are usually treated as different securities for matching purposes, but always confirm rather than assume.

The 30-day rule is one of the more mechanical parts of UK CGT and is very well established (in place since 1998), so any planning around crystallising a loss should route through Bed and ISA, Bed and SIPP, or simply accepting the 30-day gap, rather than trying to work around it directly.