Self Assessment Deadlines & Penalties

1. The Self Assessment Calendar

Four dates govern Self Assessment compliance. Each has its own penalty consequence, and HMRC's systems are fully automated, miss one and a penalty letter is triggered within days of the deadline passing.

• 5 October (post tax year): register with HMRC if this is your first Self Assessment. Missing this triggers a "failure to notify" penalty of up to 100% of the tax.
• 31 October (post tax year): paper return deadline. Most filers ignore this and file online instead.
• 31 January (following): online return deadline, balancing payment, and first Payment on Account.
• 31 July: second Payment on Account.

2. Late Filing Penalty Ladder

The late-filing regime for non-MTD Self Assessment is a four-tier escalating ladder, applied automatically regardless of whether any tax is owed. A filer with a £0 liability who submits late still pays every penalty tier.

• 1 day late: £100 fixed penalty.
• 3 months late: daily penalties of £10 begin, capped at £900 (90 days).
• 6 months late: further penalty of £300 or 5% of the tax due, whichever is higher.
• 12 months late: another £300 or 5% charge. In deliberate concealment cases, this can rise to 100% of the tax due.

A return filed 12 months late on a £20,000 liability incurs £100 + £900 + £1,000 + £1,000 = £3,000 in filing penalties alone, before late-payment penalties and interest are added. The total cost can comfortably exceed £5,000 on a moderate liability.

3. Late Payment Penalties

Late-payment penalties apply to the balancing payment and to Payments on Account that remain unpaid for an extended period. Unlike filing penalties, they only apply to outstanding tax, a fully paid filer who missed only the filing deadline does not incur these charges.

• 30 days late: 5% of the outstanding tax.
• 6 months late: further 5%.
• 12 months late: further 5%.

4. The MTD Points-Based System

MTD for ITSA filers operate under a different regime, aligned with MTD for VAT. Each late quarterly update or Final Declaration earns one compliance point. No monetary penalty is charged until the points threshold is reached, which rewards filers who are occasionally late but punishes habitual offenders.

• Threshold: 4 points in a rolling 24-month period.
• Triggered penalty: £200 on the fourth point, then £200 for every subsequent late submission until a clean period resets the record.
• Reset period: 24 months of perfect compliance clears all points.

Late-payment penalties under MTD are harsher and more incremental than the legacy Self Assessment regime. A 2% penalty applies after 15 days, rising to 4% after 30 days, then a daily 4% annualised accrual. Interest continues alongside at base rate + 4%.

5. Interest on Unpaid Tax

Interest is charged on any tax paid after its due date at the Bank of England base rate plus 4%. This applies to the balancing payment, Payments on Account, and any penalty amounts that remain unpaid. Interest accrues daily and is not tax-deductible against future profits.

HMRC also pays interest, known as "repayment supplement", on tax refunded to you after being held too long, but the rate (base minus 1%) is markedly less generous than the rate HMRC charges. The asymmetry is a deliberate policy lever against late payment.

6. Reasonable Excuse Appeals

Every penalty can be appealed within 30 days of issue on the basis of "reasonable excuse", an event that prevented compliance despite the filer taking reasonable care. HMRC's definition is narrow but genuine: serious illness, bereavement of a close relative, technology failures within HMRC's own systems, and documented postal service failures have all been accepted in tribunal cases.

What HMRC rejects outright: pressure of work, reliance on an accountant who failed to file, forgetting the deadline, not receiving a reminder, and difficulty understanding the rules. The test is whether a responsible taxpayer exercising reasonable foresight and due diligence would have been in the same position.

Successful appeals require documentation, medical certificates, death certificates, IT support tickets, postal receipts. Unsupported assertions rarely win. An appeal must be submitted through form SA370 or online, and the tax and penalties must still be paid (or Time-to-Pay agreed) while the appeal is considered.

7. Time-to-Pay Arrangements

A Time-to-Pay arrangement allows Self Assessment liabilities up to £30,000 to be spread over 12 months by direct debit, set up online without speaking to HMRC. Larger debts require a phone conversation with HMRC's Payment Support Service and a more detailed affordability conversation.

Critical point: Time-to-Pay must be agreed before the 31 January deadline to prevent late-payment penalties. Agreeing an arrangement on 1 February preserves no penalty relief for the previous day. Interest continues to accrue throughout the arrangement, but no penalty is added provided the schedule is honoured.

If you are already late or facing a penalty you believe is unjust, see our sole trader accounting service, we prepare structured appeals and manage Time-to-Pay negotiations.