The date that decides whether you owe HMRC nothing extra or start clocking up interest — and why the January bill is so often bigger than people expect.
For most self-employed people the whole tax year funnels down to one date: 31 January. It is the day your tax actually has to be in HMRC's account, and it is the day the system springs its most common surprise — because it asks for two things at once. Understanding what falls due, and when, is the difference between a planned payment and a scramble that ends in interest charges.
Self Assessment has two payment dates in a normal year. The big one is 31 January, which sits at the end of the tax year that closed the previous 5 April. The smaller one is 31 July, which carries the second payment on account. The reason January feels heavy is that it bundles together two distinct charges.
The first is the balancing payment — the tax still owed for the year you have just filed. The second is the first payment on account for the year now underway, an advance instalment equal to 50% of the prior year's liability.
Take Marcus, a self-employed photographer. His 2024/25 liability comes to £8,000, and because his tax was not collected at source and exceeds £1,000, payments on account apply. During 2024/25 he had already made two payments on account of £3,000 each (based on a smaller prior year), totalling £6,000.
Here is what falls due on 31 January 2026.
| Item | Amount |
|---|---|
| 2024/25 liability | £8,000 |
| Less payments on account already made | (£6,000) |
| Balancing payment | £2,000 |
| First payment on account for 2025/26 (50% of £8,000) | £4,000 |
| Total due 31 January 2026 | £6,000 |
Then on 31 July 2026 Marcus pays the second payment on account for 2025/26: another £4,000. Spreading for the July date through the year is exactly what trips people up — check the figure early with the late filing and payment calculator.
The deadline is when cleared funds reach HMRC, not when you press send. Faster Payments and debit card payments usually clear the same or next day, but Bacs takes about three working days and a posted cheque longer still. If 31 January falls on a weekend, HMRC must receive the money by the previous working day for some payment methods, so leaving it to the last hour is risky.
The payment deadline and the filing deadline share a date but not a fate — you can file on time and still be charged interest for paying a day late. Treat 31 January as the day the money must clear, not the day you start the return.
Miss 31 January and interest runs from 1 February on the unpaid balance. The balancing payment also attracts a 5% surcharge once it is more than 30 days late, with further 5% charges at six and twelve months. Crucially, payments on account themselves are not surcharged in the same way, but they do attract interest if paid late. If you genuinely cannot pay, contacting HMRC to arrange a Time to Pay plan before the deadline can prevent the surcharges from building.
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