UK Crypto Tax Deadlines 2026/27: Every Date That Actually Matters

Most UK crypto holders know "the deadline is end of January" and not much else. There are actually six dates that matter across the 2026/27 cycle, and missing some of them costs significantly more than missing others. Here is the full calendar plus the penalty table for getting any one of them wrong.

The full UK Self Assessment calendar — 2025/26 tax year

The 2025/26 tax year ran from 6 April 2025 to 5 April 2026. The relevant filing deadlines fall in the calendar year after the tax year ends.

**5 October 2026** — Deadline to register for Self Assessment if you have never filed before. If 2025/26 is your first year owing crypto tax, you must register by this date. Penalty for missing: nothing direct, but you can't file until you're registered, so it cascades into the late-filing penalty below.

**31 October 2026** — Deadline for paper Self Assessment returns. Almost nobody files crypto tax on paper because the schedules are complex, so practically this date is irrelevant — but if you DO want to file on paper, it's this date, not 31 January.

**30 December 2026** — Last day to file online IF you want HMRC to collect a tax owed of less than £3,000 by adjusting your PAYE tax code for 2027/28. Usually only relevant if your crypto gain is small and you have a regular salaried job.

**31 January 2027** — The big one. Deadline for online Self Assessment for the 2025/26 tax year. ALSO the deadline to PAY any tax owed for 2025/26. ALSO the first payment-on-account for 2026/27 if you're self-employed (most crypto holders aren't, but check). Filing or paying late triggers immediate penalties (see below).

**31 July 2027** — Second payment-on-account for 2026/27 if applicable.

The 2026/27 tax-year-end calendar — for the year in progress

**5 April 2027** — Last day of the 2026/27 tax year. Disposals after this date are taxed in 2027/28 instead. The annual £3,000 exempt amount resets at midnight.

**~6 March 2027** — Practical deadline for tax-loss harvesting in 2026/27. If you sell a losing position and want to claim the loss for 2026/27, you cannot re-buy within 30 days (HMRC's bed-and-breakfast rule). Selling on 6 April or later would push the loss into 2027/28 — fine if your gains are also there, but a worse outcome if you have already crystallised 2026/27 gains.

**~5 April 2027** — Last sensible day to spread purchases across spouses to use both £3,000 AEAs. Inter-spouse transfers are no-gain-no-loss but require evidence — don't leave it to 4 April.

The penalty matrix for missing 31 January 2027

The fixed penalty schedule applies regardless of how much tax you owe:

- **One day late** — automatic £100 penalty. No appeal except for "reasonable excuse" (bereavement, hospitalisation, technical failure of HMRC's site). - **Three months late** — additional £10/day penalty. After 90 days, this is £900 in daily fines plus the original £100. - **Six months late** — additional £300 or 5% of tax owed, whichever is greater. - **Twelve months late** — additional £300 or 5% of tax owed, plus potentially up to 100% of the tax owed if HMRC believes you deliberately withheld.

If you file on time but PAY late: 5% of tax owed at 30 days late, another 5% at six months late, another 5% at twelve months late, plus interest currently at 7.75% (HMRC base rate +2.5%) accruing daily.

The cheapest mistake is filing on time but underpaying. The most expensive is not filing at all.

Special crypto scenarios with their own deadlines

**Negligible value claim (s.24 TCGA)** — must be made within 4 years of the end of the tax year in which the asset became negligible. If you have a Pump.fun token that died in 2025/26, you have until April 2030 to claim the loss. Don't try to backdate.

**Carry-forward losses (CG14580)** — must be CLAIMED on the relevant year's Self Assessment to be available for future offset. You can't suddenly remember a £20,000 loss from 2023 in your 2026/27 filing — the loss had to be reported in 2022/23. If you forgot, you can amend the prior year's return within 12 months of its filing deadline (so a 2022/23 return could be amended until 31 January 2024). Once that window closes, the loss is gone.

**Inter-spouse transfers (s.58 TCGA)** — no-gain-no-loss treatment requires the transfer to occur in the same tax year as the matching disposal. Don't move BTC to your spouse on 6 April to "use their AEA against your 2025/26 gains" — too late, that AEA belongs to 2026/27.

**Disclosure of "innocent" mistakes** — if you realise after filing that you under-reported, the "unprompted" disclosure window for full HMRC enquiry runs four years from the end of the relevant tax year. For 2025/26, that means until 5 April 2030. Voluntarily disclosing through the Digital Disclosure Service is materially cheaper than waiting for HMRC to find it via CARF.

What actually catches people out

Most UK crypto filers who get caught aren't deliberately evading — they're either:

1. Not filing at all because they assumed gains under £3,000 didn't need reporting (they do, if total proceeds exceed £50,000 — even £0 gain). 2. Treating crypto-to-crypto swaps as non-events (they're disposals). 3. Forgetting that staking rewards are income at receipt FMV, not at sale. 4. Computing Section 104 wrong by not including same-day matches. 5. Missing the carry-forward window to actually claim a previously-realised loss.

Each of these is a five-figure cost for an actively trading wallet over a few years.

Practical preparation calendar

Working backwards from 31 January 2027:

- **January 2027** — File. Pay. Done. - **December 2026** — Run final reconciliation. CryptoLens generates a comparison against your exchange CSVs; resolve any gaps. - **November 2026** — First draft of SA108. Identify any disposals that need accountant-level review. - **October 2026** — Register for Self Assessment if first-time filer. Download exchange CSVs while they're still easy. - **September 2026** — Decide on tax-loss-harvest moves to make before year-end (you have until ~6 March 2027 to actually execute). - **August 2026** — Start labelling unknown wallets and tagging transactions. This is the most time-consuming step if left until January.

CryptoLens runs the SA108 calculation any time, against any combination of wallets and exchange CSVs. Run it monthly through the autumn and the January deadline becomes a formality instead of a panic.