UK Self Assessment 2026/27 — Deadlines, Penalties, Who Must File
Reviewed by Alex Morgan · Updated April 2026 · UK tax year 2026/27 (6 April 2026 → 5 April 2027)
All the dates you actually need
Self Assessment runs on a fixed calendar. Once HMRC issues your notice to file, these dates are immovable. They don't slide if 31 January falls on a weekend either — the cut-off is always end-of-day on the date itself, in your bank's clearing window.
| What | Deadline | Why it matters |
|---|---|---|
| Register for Self Assessment | 5 October 2027 | If 2026/27 is your first SA year. Miss it and you face a "failure-to-notify" penalty of up to 100% of tax owed. |
| Paper return deadline | 31 October 2027 | Almost no one files paper anymore. If you must, allow 2 weeks for HMRC postal handling. |
| Online return deadline | 31 January 2028 | 11:59 pm UK time. Submit at gov.uk/log-in-file-self-assessment-tax-return. |
| Pay tax owed for 2026/27 | 31 January 2028 | Bank transfer needs 3 working days. Faster Payments same day. Direct Debit needs setup 5 days ahead. |
| First payment on account 2027/28 | 31 January 2028 | 50% of your 2026/27 bill (if >£1,000). Reduces the 31 Jan 2029 balancing payment. |
| Second payment on account 2027/28 | 31 July 2028 | Other 50%. Common cashflow trap for newly self-employed in their second year. |
Who must file Self Assessment 2026/27?
HMRC's official list is long but the criteria that catch most people are:
- Self-employed with gross trading income above £1,000 (the Trading Allowance)
- Partner in a partnership — file the partnership return AND your own personal return
- Landlord with rental income above £1,000 (Property Allowance) or net rental profit at all
- Dividend income above £500 (Dividend Allowance fell from £1,000 to £500 in April 2024)
- Untaxed savings interest above the Personal Savings Allowance (£1,000 basic-rate, £500 higher-rate, £0 additional-rate)
- Capital gains above the £3,000 Annual Exempt Amount
- Foreign income of any meaningful amount
- High Income Child Benefit Charge (HICBC) — if you or partner earns over £60,000 and someone in the household claims Child Benefit
- Total income above £150,000 (raised from £100,000 in 2023/24 — fewer PAYE-only earners now have to file)
- HMRC sent you a notice to file — even if you don't think you owe tax, you must file or formally request the notice be withdrawn
The Trading Allowance and Property Allowance let you earn up to £1,000 each from side hustles or rental without registering for Self Assessment. If you go over either by £1, you have to file a full return for the entire year — not just the bit above £1,000.
The penalty schedule (this is the bit that bites)
| How late | Filing penalty | Payment penalty (in addition) |
|---|---|---|
| 1 day | £100 fixed (charged automatically) | Daily interest on tax owed (HMRC base rate + 4%) |
| 3 months | £10/day for up to 90 days (max £900) — on top of the £100 | Daily interest continues |
| 6 months | 5% of tax owed or £300 (whichever is more) | 5% surcharge on unpaid tax |
| 12 months | Another 5% or £300 (whichever is more) — could go higher if HMRC suspects deliberate late filing | Another 5% surcharge |
Worked example: file 12 months late with £5,000 tax owed and you've stacked up £100 + £900 + £300 + £300 in filing penalties (£1,600), plus £250 + £250 in late-payment surcharges (£500), plus around £600 in interest. £2,700 in penalties and interest on a £5,000 bill — over half of what you originally owed.
How payments on account work (and why they hurt year 2)
If you owe over £1,000 and less than 80% was deducted at source (PAYE/CIS), HMRC asks for tax on next year in two instalments. Year 1 of self-employment, you owe one year's tax on 31 January. Year 2, you owe last year's balancing payment plus 50% on account for the current year — both due the same day.
A self-employed worker with £40,000 profit owes around £6,000 income tax + £3,000 Class 4 NI = £9,000 in year 1. In year 2 they owe their year 1 balancing payment + 50% on account for year 2 (~£4,500) = ~£13,500 due 31 January. Then £4,500 more on 31 July. Plan cashflow now or it becomes an emergency in 18 months.
Common Self Assessment traps
- HICBC for high earners with kids — £60,000+ income forces you onto Self Assessment even if PAYE. Most don't realise.
- Side hustles over £1,000 — selling on Vinted, eBay, Etsy. HMRC now receives platform data automatically (UK Digital Platform Reporting rules from 2024).
- Rental "profit" — interest restriction means landlords with mortgages often have higher taxable profit than cash profit.
- First-year payments-on-account shock — see above.
- Foreign dividends — most people forget. £100 in US dividends paid via Trading 212 still goes on the return.
- Filing on phone Wi-Fi at 11:55 pm — HMRC's portal genuinely struggles with the volume on the night. Submit by 11 pm to be safe.
Pair this with
- → UK Take-Home Pay Calculator — model your tax bill before 31 January
- → UK IR35 2026 explained — for inside-IR35 contractors filing alongside PAYE
- → UK 60% tax trap 2026/27 — pension contributions to dodge it before year-end
- → UK Marriage Allowance 2026 — claim £252 + £1,008 backdate via Self Assessment
- → UK Statutory Rates 2026/27 (open dataset) — every threshold cited above