Self Assessment deadline (31 January)

The Self Assessment return (form SA100) for the tax year ending 5 April is due online by 31 January the following year. The balancing payment and the first payment on account are also due by this date.

What is this filing?

Self Assessment is the UK system for reporting personal income, gains, and tax for self-employed individuals, higher-rate taxpayers, landlords, and others. Paper returns have an earlier 31 October deadline; the 31 January date is for online filing.

When is it due?

Online return and balancing payment: 31 January following the tax year. Paper return: 31 October. First payment on account: 31 January. Second payment on account: 31 July.

What's the penalty for missing it?

Late filing: £100 immediate, £10/day after 3 months (max £900), 5% of unpaid tax or £300 (whichever greater) after 6 months, same again after 12 months. Late payment: interest accrues from 1 February; first 5% surcharge on amounts still unpaid at 3 March (30 days after the 31 Jan deadline); further 5% at 6 months and 12 months.

What it actually costs

The combined SA penalty exposure is severe: £100 on day one, daily penalties of up to £900 from month three, then 5% of unpaid tax (or £300, whichever is higher) at six months and again at twelve months. For a client with £50,000 unpaid at 31 January, the six-month penalty alone is £2,500. Late-payment interest at HMRC's current rate (base rate plus 4%) adds further cost from 1 February. A client six months late faces the immediate return penalty, daily penalties, a 5% late-payment surcharge, and months of interest — all simultaneously.

Why this catches practices out

• The 31 January deadline falls during the busiest week of the accountancy calendar — a single technical issue with HMRC's online service can cascade across a full client book.
• Clients who receive HMRC's 'notice to complete a return' late still face a 31 January deadline regardless of when the notice arrived.
• Payments on account are due on the same date as the return — clients who expected only a balancing payment are surprised by the combined liability.
• HMRC's £10/day penalty (from 3 months late) accumulates silently to £900 before a further 5% penalty kicks in at 6 months.

How PenaltyProof helps

PenaltyProof tracks the 31 January deadline at practice level. Two-track Self Assessment monitoring (primary + secondary dates like 31 July and 5 October) is available on paid plans.

Frequently asked questions

What is the penalty for filing one day late on 1 February?

£100 immediate fixed penalty, regardless of whether any tax is due or whether the return is only one day late.

When does the £10/day penalty start?

After the return is 3 months late (i.e. from 1 May for a January deadline). HMRC can charge up to 90 days of daily penalties, capped at £900.

Is the payment deadline the same as the filing deadline?

Yes — the balancing payment and the first payment on account are both due 31 January alongside the return. Interest runs from 1 February on any unpaid tax; the first late-payment surcharge (5%) falls 30 days after 31 January.

Does PenaltyProof cover the paper SA return deadline of 31 October?

Not currently — the 31 October paper deadline is a practice-level alert. PenaltyProof focuses on the headline 31 January and secondary payment dates.

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