The 30-day rule is the single most misunderstood part of UK Capital Gains Tax. It’s not complicated once you understand it, but the number of people who get it wrong \u2014 and the tax consequences \u2014 are significant. Here are the seven mistakes I see most often.<\/p>\n
The rule matches your sale against purchases made in the 30 days after<\/strong> the sale. Not before. Shares bought before the sale date are already sitting in your Section 104 pool \u2014 they’re not affected by the 30-day rule.<\/p>\n This is counterintuitive. You’d think “I bought these shares a week ago and now I’m selling them” would trigger something. It doesn’t. The 30-day rule only looks forward from the sale date.<\/p>\n The logic makes sense once you understand the purpose: HMRC wants to prevent you from selling shares to create a loss and then immediately buying them back. If you sell on Monday and rebuy on Wednesday, the rule catches that repurchase \u2014 because you’re essentially pretending you sold when you didn’t really.<\/p>\n The 30-day window is 30 calendar days<\/strong>, not 30 trading days. Weekends, bank holidays, and Christmas all count. If you sell on 1 March, the window runs until 31 March (inclusive). A purchase on 31 March is caught. A purchase on 1 April is not.<\/p>\n People who think in trading days often assume they have longer than they actually do.<\/p>\n This is probably the most common accidental trigger. You sell some shares, and then your monthly investment plan kicks in two weeks later and auto-buys the same stock.<\/p>\n That automatic purchase falls within the 30-day window. The rule doesn’t care that you didn’t manually place the order \u2014 the purchase exists, so it matches.<\/p>\n If you’re planning to sell a holding to crystallise a gain or loss, pause your regular investment in that stock first. Wait for the 30-day window to close, then restart the auto-invest.<\/p>\n Actually, this one people get wrong in the other direction. ISA purchases do not<\/strong> trigger the 30-day rule. The rule only applies to acquisitions in the same capacity \u2014 meaning your GIA. Buying the same shares inside an ISA within 30 days is perfectly fine. This is exactly why “bed and ISA” works as a tax planning strategy.<\/p>\n However, buying back in your GIA within 30 days does trigger the rule.<\/p>\n If multiple purchases fall within the 30-day window, the earliest<\/strong> one matches first. Not the biggest, not the closest in quantity \u2014 the first one chronologically.<\/p>\n Example: You sell 500 shares on 1 Feb. You buy 200 on 10 Feb and 300 on 20 Feb. The first 200 sold match to the 10 Feb buy. The next 300 match to the 20 Feb buy. Each portion has its own cost basis and gain\/loss.<\/p>\n Getting the order wrong changes your tax numbers.<\/p>\n The 30-day rule applies across all your accounts (excluding ISAs). If you sell Tesla on Trading 212 and buy Tesla on Robinhood 12 days later, the rule triggers. HMRC doesn’t care about which platform the trades happened on \u2014 it’s the same person buying the same shares.<\/p>\n People who split their trading across multiple brokers sometimes miss this because each broker’s records show an unrelated-looking transaction.<\/p>\n The 30-day rule doesn’t eliminate your loss \u2014 it defers it. If you sell at a loss and rebuy within 30 days, the loss gets matched to the new purchase rather than being an allowable loss right now. But those new shares now have a cost basis that reflects the higher original purchase price. When you eventually sell them (outside of any further 30-day window), the loss crystallises then.<\/p>\n So the loss isn’t gone \u2014 it’s just pushed into the future. The issue is that you can’t use it to offset gains in the current tax year, which might have been the whole point of selling.<\/p>\n The simplest approach: if you want to sell shares and don’t want the 30-day rule to complicate things, don’t buy the same shares (in any GIA account) for 31 days after the sale. Set a calendar reminder.<\/p>\n If you need to stay invested in that sector, buy a similar but different investment. Sold a FTSE 100 tracker? Buy a FTSE 250 tracker. They’re different securities \u2014 no 30-day match.<\/p>\n If you want to sell and immediately rebuy, do it via a “bed and ISA” \u2014 sell in GIA, buy in ISA. The 30-day rule doesn’t apply to ISA acquisitions.<\/p>\nMistake 2: Not counting weekends and holidays<\/h2>\n
Mistake 3: Forgetting that regular investments trigger it<\/h2>\n
Mistake 4: Assuming ISA purchases don’t count<\/h2>\n
Mistake 5: Not matching against the earliest buy<\/h2>\n
Mistake 6: Ignoring the rule on cross-broker purchases<\/h2>\n
Mistake 7: Thinking the rule destroys your loss<\/h2>\n
How to avoid problems<\/h2>\n