For UK crypto investors before May 2027
CARF 2027.
Get ahead of what HMRC will see.
UK CARF reporting started 1 January 2026. Exchanges file their first reports with HMRC by 31 May 2027 — including data on UK-resident users directly. The practical question for every UK investor is whether your Self Assessment will reconcile against what your exchange reports.
Free up to 200 transactions. Pay only when you download the report.
1 Jan 2026
UK CARF live
Around 50 UK-serving exchanges began collecting CARF-shaped data on customer activity. The reporting period now runs.
31 May 2027
first reports to HMRC
The statutory deadline for UK-serving exchanges' first reports. Data on UK users of overseas exchanges follows via international exchange from later in 2027.
Lower penalties
if you act first
Voluntary correction before HMRC raises the question consistently lands in a lower penalty band than being asked. The reconciliation is cheapest done early.
Why CARF matters for your return
Two pictures that need to match
Exchanges report gross activity. Every buy, sell, swap, deposit, and withdrawal is sent to HMRC with your name, address, and tax identifier attached. There is no UK tax logic in that feed.
Your Self Assessment shows the net taxable position — after Section 104 pooling, after same-day and 30-day matching, after transfers between your own wallets, after DeFi events the exchange never saw, and after gas fees on disposals.
Those two pictures should describe the same person. When they don't — because gross activity looks much larger than net disposals, or because the exchange-side history references years you didn't file for — HMRC can open an enquiry. Reconciling now means you either confirm everything ties up, or you find the gap and correct it before the question lands in your inbox.
Which path is mine?
Three clear routes
Pick by transaction count and activity type. Most retail investors land in the first column.
Most common
Under 10,000 transactions
Coinbase, Binance, Kraken plus some DeFi. Run it yourself.
- • Connect wallets + import exchange CSVs
- • Engine sorts every event under HMRC rules
- • SA108-ready PDF + CSV download
- • £49 (Light) or £99 (Active) per tax year
Above the cap
Over 10,000 transactions
High-volume DeFi, multi-wallet, NFT activity. We run the sync for you.
- • Email your wallet addresses
- • Fixed-price quote within 48 hours
- • Same engine, same audit trail
- • Typically £249–£449 per tax year
Specialist needed
Trade or business activity
Mining as a trade, company crypto, large undisclosed positions.
- • Different tax treatment from CGT
- • Often needs SA100 income side too
- • A specialist accountant runs it end-to-end
- • We can refer you
Already received an HMRC nudge letter?
The 60-day clock is the binding constraint, not CARF. Start at /hmrc-letter — the same self-serve / Concierge / accountant routing, shaped around the deadline rather than the readiness window.
If self-serve fits
What you get from ChainTax
CEX and DeFi in one Section 104 pool
The reconciliation only works if exchange disposals and on-chain disposals share the same average-cost pool. Most spreadsheets and single-surface tools split them — ChainTax pools them.
Same-day → 30-day → S104, in HMRC order
The matching rules HMRC requires, applied per disposal. 2024/25 split-year CGT (10/20% before 30 Oct, 18/24% after) handled automatically.
Show Working — full audit trail
Every disposal records the matching rule, the S104 pool snapshot before and after, and the price source with confidence rating. If HMRC asks for working under a CARF-driven enquiry, you have it.
Multi-year reconciliation
Each tax year is a separate report; loss carry-forward is automatic across years. Bundle 2 years for a £19–£29 saving on a multi-year reconciliation.
How it works
Four steps, mostly waiting on the sync
- 1
Sign up and add your wallets
Public addresses only — never private keys or exchange API keys. Add Coinbase, Binance, or Kraken via CSV import in the same flow.
- 2
The engine sorts every transaction
Disposals, income, transfers, liquidity — across Ethereum, Arbitrum, Optimism, Base, and Polygon. 33 protocol-specific detectors plus pattern matching and a generic-swap fallback.
- 3
Compare against what you filed
The summary surfaces total disposals, taxable gains, income, and SA108 box mapping per year. Hold those next to your filed Self Assessment for each open year and the reconciliation is direct.
- 4
Confirm or amend, before May 2027
If the figures match, no action needed. If they don't, the PDF + CSV gives you the working to amend a return or file a voluntary disclosure with full evidence.
An honest note on CARF specifics
What's confirmed and what's still settling
Confirmed
- • UK CARF live from 1 January 2026 (SI 2025/744, final law)
- • First exchange reports due to HMRC by 31 May 2027
- • Data schema: the OECD CARF XML schema
- • ~50 UK-serving exchanges in scope
- • DeFi: control-based test — genuinely autonomous protocols have no reporting entity
- • HMRC penalty bands and look-back windows unchanged
Still settling
- • Precise timing of overseas-exchange data reaching HMRC (international exchange runs from later in 2027)
- • Whether reporting expands to controlled DeFi front-ends in later phases
- • How HMRC will operationally cross-match returns
The reconciliation itself doesn't depend on those open questions. Your Self Assessment either reflects the activity HMRC can see, or it doesn't — that's the work, and it's the same work whichever schema lands.
We update this page as official guidance is published.
What ChainTax doesn't do yet
No tax tool is perfect. Here's what ChainTax cannot yet handle automatically — and what you should check manually or discuss with your accountant.
- •Aave interest (aTokens) accrues via balance changes rather than individual transactions, so it doesn't appear in your transaction history. For now, add aToken interest manually as an income event — automatic detection via daily balance snapshots is on the roadmap.
- •Lido stETH rebase rewards happen at the protocol level and rarely appear as transactions in your wallet — so ChainTax now detects them automatically from your stETH balance growth and books each as income at its market value on the date received. It covers every stETH wallet on your account — add another and your next sync recalculates your rebase rewards across the complete set.
- •Deposits into Aave, Compound, Yearn, EigenLayer, and similar fixed-ratio lending or staking protocols are treated as transfers, not disposals — your cost basis carries to the receipt token (aToken, cToken, vault share). HMRC's strict reading (Cryptoassets Manual CRYPTO61620) could treat these as deemed disposals at deposit; HMRC confirmed in November 2025 that it is developing no-gain-no-loss rules matching the treatment ChainTax applies today, but they are not yet law — if you want strict-reading compliance, override the deposit event manually.
- •Some protocols return ETH via internal calls (Nexus Mutual, Rocket Pool, cbETH). We catch most of these automatically; when we can't, we flag the event for your review.
- •Five EVM chains supported: Ethereum, Arbitrum, Optimism, Base, and Polygon. Other chains (Solana, Avalanche, etc.) are not yet covered.
Full detail: How it works → Known limitations
FAQ
Common questions
What is CARF and why does May 2027 matter?
The Crypto-Asset Reporting Framework (CARF) is an OECD-designed regime that requires crypto exchanges to report customer activity to tax authorities. The UK activated CARF on 1 January 2026, and the implementing regulations are now final law (SI 2025/744). UK-serving exchanges must file their first reports — covering 2026 activity — with HMRC by 31 May 2027, including data on UK-resident users directly. Data on UK users of overseas exchanges follows via international exchange between tax authorities from later in 2027. From that point forward, HMRC holds a structured feed of exchange-side records — names, addresses, tax IDs, and transaction history — that it can match against your Self Assessment. The 2026/27 return, due 31 January 2028, is the first one HMRC can cross-check against CARF data.
Which exchanges and platforms are covered?
CARF applies to centralised exchanges and custodial services that operate in or serve UK customers — Coinbase, Binance, Kraken, and around 50 UK-serving providers in total. The final UK rules use a control-based test rather than a blanket DeFi carve-out: a protocol or front-end with an identifiable controlling business can be a reporting entity, while genuinely autonomous protocols (Uniswap, Aave, Curve, etc.) have no one to file the report. In practice most DeFi activity stays outside the feed. However, exchange deposit and withdrawal records still let HMRC link wallets back to identified accounts, and on-chain activity is fully traceable from there.
What if my exchange filings don't match what I filed?
This is the central readiness question. Exchanges report gross activity — every buy, sell, swap, deposit, and withdrawal — without applying UK tax rules. Your Self Assessment should reflect the same activity after Section 104 pooling, same-day and 30-day matching, transfers between your own wallets, and DeFi events the exchange never saw. If those two pictures don't reconcile, HMRC may open an enquiry. Recomputing your gains under HMRC rules now lets you confirm or amend your return before the data arrives.
What should I actually do before May 2027?
Pull your full transaction history across exchanges and wallets, recompute the gains under Section 104 pooling, and compare against the figures already on your Self Assessment for open tax years. If everything reconciles, no action is needed. If you find missed disposals, unreported income, or under-stated gains, voluntary correction or disclosure before HMRC raises the question typically lowers the penalty band by a wide margin. Coming forward unprompted is consistently treated more leniently than being asked.
How far back can HMRC go once they have CARF data?
HMRC's standard look-back is four years for innocent error, six years for careless behaviour, and up to twenty years for deliberate non-disclosure. CARF doesn't change those windows — it gives HMRC much better source material for opening an enquiry within them. Most retail investors' full crypto history sits inside the look-back, which is why the practical question is reconciliation across all years on file, not only 2026 onwards.
Does ChainTax cover the DeFi side that CARF doesn't?
Yes. CARF covers exchanges. ChainTax covers exchanges and DeFi together — 33 protocol-specific detectors across Ethereum, Arbitrum, Optimism, Base, and Polygon, plus CSV imports for Coinbase, Binance, Kraken, and Koinly. The same Section 104 pool spans CEX and on-chain disposals, which is the part most tools and spreadsheets get wrong.
How much does it cost to get a CARF-ready report?
Free up to 200 transactions. £49 for Light (up to 2,500 per tax year) or £99 for Active (up to 10,000). One-time payment per tax year — if you need multiple years for a clean reconciliation, you pay per year, with a 2-year bundle option. Concierge sync (we run it for you, over 10k transactions) is quote-first, typically £249–£449.
What's not yet public about CARF in the UK?
Less than there used to be. The implementing regulations are final law (SI 2025/744), the data schema is the OECD's CARF XML schema, and the statutory deadline for exchanges' first reports to HMRC is 31 May 2027 — all confirmed. Still settling: exactly how HMRC will operationally cross-match the feed against Self Assessment returns, the precise timing of data arriving from overseas exchanges (international exchange runs from later in 2027), and whether reporting expands to controlled DeFi front-ends in later phases. We update this page as guidance lands.
Background reading: the full guide to CARF and what it means for UK investors.
Reconcile now, while voluntary correction is still on the table
Free up to 200 transactions. Pay only when you're ready to download the report. No subscription, no auto-renewal.
ChainTax is a tax calculation tool and does not provide tax, financial, or legal advice. Verify your figures with a qualified adviser before filing.