CARF and Self Assessment
Crypto Self-Assessment Mismatch: HMRC Letters, CARF Data & Tips
HMRC Will Have Your Exchange Data From 1 January 2026. Will It Match Your Tax Return?
From 1 January 2026, UK crypto platforms must collect and report your identity and transaction data to HMRC under the Cryptoasset Reporting Framework (CARF). The first reports land with HMRC by 31 May 2027, covering the whole of the 2026 calendar year. If the numbers HMRC receives don't line up with what you filed, that's a mismatch, and HMRC won't need to guess at it.
We reconcile your full trading history, recalculate your position properly, and correct your filings before HMRC opens the file.
The mismatch
What A Crypto Self Assessment Mismatch Actually Is?
Your exchange doesn't report your profit. It reports your activity: total disposals, gross proceeds, wallet addresses, and your identifying details, including your National Insurance number.
So HMRC's copy of your year might show £180,000 of disposals across four platforms. Your return shows a £2,100 gain, or nothing at all. Both figures can be entirely correct, because gross proceeds and taxable gains are different things. But HMRC can only see one side of that until someone explains the other. That gap is the crypto Self Assessment mismatch HMRC systems are built to flag.
It isn't an accusation. It's a question you now have to answer with evidence.
Patterns we see
The Mismatches We See Most Often
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You Declared The Cash-Out, Not The Trades
You reported the £40,000 that hit your bank. But every crypto-to-crypto swap on the way there was a disposal, priced in sterling on the day. HMRC's report shows the disposal count; your return shows one figure.
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One Exchange Never Made It Into The Calculation
Coinbase and Kraken were reconciled carefully. The platform you used for three months in 2021 wasn't, because it's offshore or it collapsed. CARF is international, and defunct exchanges leave records in administration, in your inbox, and on-chain.
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Staking And Airdrop Rewards Were Treated As Free Money
Most staking, lending, and airdrop receipts are income at the sterling value on the day they land, and that value becomes your cost basis. Skip it, and you have understated income and overstated the later gain: one omission, two errors.
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The Cost Basis Was Averaged Instead Of Pooled
Total spend divided by total coins gives a tidy average and the wrong answer. HMRC requires same-day matching, then the 30-day rule, then Section 104 pooling, and the gap between the two methods compounds across every later disposal.
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Wallet Transfers Were Counted As Sales
Moving coins to your own hardware wallet isn't a disposal, but trackers log it as a sale plus a repurchase because they see only one side. The phantom gains run into five figures. This one usually ends in a refund.
What changed
Why This Isn't The Same As The Old Nudge Letters
HMRC has written to crypto holders before, using data obtained from a handful of exchanges on request. That was slow, partial and easy to miss.
CARF is automatic, annual and cross-border. Platforms collect your tax residence and National Insurance number at onboarding, then file with HMRC every year without anyone asking. Once that feed is running, a crypto Self Assessment mismatch HMRC identifies takes no investigative effort at all. It's a data comparison.
Penalties
Correcting It Now Costs Less Than Correcting It Later!
This is the part worth understanding clearly. HMRC treats a disclosure you make voluntarily very differently from one you make after being contacted.
Unprompted disclosures attract substantially lower penalties, and in cases of genuine error with a reasonable excuse, penalties can be reduced to nil. Once HMRC has written to you, the same underlying tax is still due, but the penalty range moves against you and interest keeps accruing throughout.
The first CARF reports arrive in 2027 for the 2026 year, and HMRC's existing data on earlier years hasn't gone anywhere. Fixing a crypto Self Assessment mismatch HMRC hasn't yet raised is a straightforward piece of compliance work. Responding to one after the fact is an enquiry.
Our process
How We Fix the Crypto Self-Assessment Mismatch
Every crypto Self Assessment mismatch HMRC could raise comes down to three questions: what did you actually do, what does that mean in tax terms, and how is it presented to HMRC? We answer all three.
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1. We Rebuild The History You Can't Access
Every exchange, wallet, DeFi protocol, and bridge worked from API exports and on-chain data rather than screenshots. That includes the platform that closed and the account you're locked out of.
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2. We Recalculate It Properly
Pooled cost basis under the correct matching rules, income separated from capital, losses claimed where they've been missed. The corrected position is often smaller than clients expect, and sometimes it's a refund.
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3. We Correct It And Handle HMRC
Returns still in date get amended. Older years go into a voluntary disclosure. From that point, the correspondence comes to us, not you.
Most engagements are completed in two to four weeks, on a fixed fee agreed before we start.
Next step
Get Ahead Of The Data!
If you have traded crypto and you are not certain your returns reflect it, the cheapest moment to deal with that is before HMRC's letter. We handle disclosures, amendments and enquiries for UK crypto holders every week.
Questions
People Also Ask
Does HMRC know about my crypto if I never told them?
Often, yes. HMRC has pulled exchange data through information requests for years, and from 1 January 2026 UK platforms report automatically under CARF. Assume anything on a KYC'd exchange is visible. Private wallets aren't reported, but transfers to and from an exchange are.
Why does my crypto exchange want my National Insurance number?
CARF requires it. Platforms must verify each user's tax residence and tax reference, which in the UK is your NI number. It's how HMRC matches the report to your Self Assessment record. Withholding it restricts your account, not your data.
What happens if HMRC finds a mismatch between my exchange data and my tax return?
You will usually get a nudge letter giving you 30 to 60 days to check your position, not an immediate enquiry. Ignoring it escalates things. Reconcile the real figures first, then reply with a calculation you can stand behind.