Declare Crypto to HMRC: The Voluntary Disclosure Guide

Declare crypto to HMRC: the voluntary disclosure guide. If you have sold, swapped, staked or earned crypto in past years and didn't report it — usually because nobody told you that a token-to-token swap is a taxable disposal — coming forward yourself is almost always the cheapest way out. HMRC runs a dedicated Cryptoasset Disclosure Service for exactly this, and the penalties for an unprompteddisclosure are a fraction of what you'd face if HMRC reaches you first.

The timing matters. Exchanges started collecting your data on 1 January 2026under the Crypto-Asset Reporting Framework (CARF). HMRC doesn't receive the first reports until 31 May 2027and won't reconcile them against Self Assessment until then — so right now, the window to disclose on your own terms is still open. Once HMRC matches that data, any contact becomes a prompted disclosure and your penalty band jumps.

This guide walks through the disclosure process step by step: notifying HMRC, the 90-day clock, the penalty bands, how many years you have to go back, and — the part most people get stuck on — how to produce prior-year figures that actually hold up.

What the Cryptoasset Disclosure Service is

HMRC's Cryptoasset Disclosure Service is a formal route to tell HMRC about unpaid Capital Gains Tax on disposals and Income Taxon rewards from past tax years. It's the right channel when you've already missed the amendment window for a return (you can normally amend a return only within 12 months of its filing deadline) or never filed at all for those years.

The point of disclosing voluntarily is the penalty. An unprompted disclosure— one you make before HMRC contacts you about it — sits in the lowest part of every penalty band. A prompted disclosure, made only after HMRC has nudged or opened an enquiry, sits higher. Same tax, very different penalty, decided entirely by who moved first.

The process: notify, 90 days, then pay within 30

The disclosure runs on a fixed set of deadlines. Miss them and you can lose the favourable treatment, so plan your figures before you start the clock.

The 90-day window is generous but it disappears fast once you realise what reconstructing several years of swaps, LP moves and rewards actually involves. The smart sequence is to get your numbers substantially ready first, then notify, so the 90 days is for finishing and submitting rather than starting from scratch.

How many years you have to disclose

You don't disclose “everything forever.” The number of years depends on whythe tax went unpaid — HMRC's behaviour-based lookback windows:

Most UK investors disclosing crypto fall into the first two rows: they genuinely didn't know that swapping ETH for USDC, removing liquidity, or selling on a centralised exchange were all disposals. That usually points to a 4- or 6-year disclosure rather than the full 20. Choosing the right band honestly matters — understating your behaviour to shorten the window is the kind of thing that turns a careless error into a deliberate one.

Penalties, interest and why unprompted wins

The disclosure total is three things stacked: the unpaid tax, interest from the original due date, and a penalty. The penalty is where coming forward early pays off.

On top of the penalty, HMRC charges interest from the date the tax was originally due — currently around 7.75%a year. Interest is not a penalty and isn't reduced by disclosing; it simply compounds the longer the tax stays unpaid, which is another reason not to wait.

Getting your figures right — the hard part

A disclosure is only as good as the numbers in it. HMRC expects gains calculated their way, not a back-of-envelope “bought at X, sold at Y.” That means:

• Section 104 pooling— every token pooled at weighted-average cost across all your wallets and exchanges, not FIFO. See the Section 104 pool explained.
• Same-day then 30-day Bed & Breakfast matching, applied in HMRC's priority order before the pool — the anti-avoidance rules that override the pool.
• Income separated from capital gains.Staking and DeFi rewards are income at market value on receipt (20/40/45%) and go on the SA100 ‘Other income’ section — not SA108. Disposals are CGT (18%/24% from 30 October 2024) and go on SA108 boxes 13.1–13.8. See DeFi income vs capital gains for where each line lands.
• Losses claimed.Capital losses in a disclosure year reduce the gains you owe tax on — don't leave them out. How crypto loss relief works.
• Non-disposals left alone. Bridging between chains and wrapping (ETH to WETH) are not taxable disposals — the cost basis carries across. Booking them as sales inflates the disclosure and overpays.

One trap worth flagging: the DeFi no gain, no loss treatment for liquidity provision was confirmed in direction at the Autumn Budget 2025, but it is not yet law— there's no draft legislation, and it is prospective, not retrospective. For the years you're likely disclosing (2024/25 and 2025/26), the strict rule still applies: an LP add is a disposal (CRYPTO61620). Don't apply the future rule to a past year.

Remember the £50,000 trigger, too: if your gross disposal proceedsin a year exceed £50,000 you had to report the disposals even if no taxable gain arose — a distinct obligation from the £3,000 annual exempt amount. A non-filed year over that threshold is exactly the kind of thing a disclosure cleans up.

How ChainTax makes a disclosure defensible

The reason most people put off a disclosure is the figures — reconstructing years of on-chain activity by hand is brutal, and an exchange's own export only covers that one platform. ChainTax solves the bottleneck:

1. Full history from genesis.Paste a wallet and the engine pulls every on-chain transaction back to the very first acquisition, across Ethereum and the major L2s — so your prior-year cost basis is built on complete data, not a partial window.
2. One cross-platform Section 104 pool. On-chain activity and centralised-exchange CSVs (Coinbase, Binance, Kraken) feed into a single pool per token, the way HMRC requires.
3. Deterministic DeFi classification. Swaps, LP adds/removes, bridges, wraps and staking rewards are each handled as a disposal, transfer or incomeper the rules — not bucketed as phantom gains the way most crypto tax tools do.
4. Per-year SA108 figures with the working shown.Each disposal carries its matching rule, pool snapshot, cost basis and price source — the audit trail that lets you (or an accountant) stand behind the numbers in a disclosure.

You can calculate your prior-year crypto CGT free for up to 200 transactions, see the full report preview, and only pay when you download the SA108-ready figures for the years you need to disclose. For the wider picture on what HMRC can actually see, our guide to whether HMRC can see your crypto and the CARF reporting timeline set out exactly why the unprompted window is closing.