Why your first tax bill is bigger than you expected.

Almost every new sole trader has the same moment: the first Self Assessment bill arrives and it is much bigger than the tax they worked out on their profit. Nothing has gone wrong. It is a timing rule called payments on account, and once you see how it works, it never surprises you again.

The rule

If your Self Assessment bill is more than £1,000 (and less than 80% of your tax is collected at source, through PAYE for example), HMRC assumes next year will look like this year — and asks you to pay next year's tax in advance, in two instalments:

• 31 January — this year's balance, plus half of next year's estimated bill
• 31 July — the other half of next year's estimated bill

So the first January bill is roughly one and a half times the tax you were expecting. You are not being charged extra tax — you are paying next year's tax early.

A worked example

Say your first year of trading produces a tax bill of £4,000.

• 31 January: £4,000 (the year just ended) + £2,000 (first payment on account) = £6,000
• 31 July: £2,000 (second payment on account)

The following January, your actual bill for that next year is settled against the £4,000 you have already paid — plus the next round of payments on account begins.

What you can do about it

• Put money aside from the start. A simple habit — a fixed percentage of everything you invoice moved to a separate account — absorbs the first January without drama. Ask us what percentage fits your situation.
• Reduce payments on account if profits are falling. The instalments are based on last year. If this year is genuinely quieter, you can apply to reduce them — but reduce too far and HMRC charges interest on the shortfall, so it needs a realistic forecast, not optimism.
• File early. Filing in the summer doesn't mean paying earlier — the deadlines stay the same. It means you know the exact January number months in advance.

Where people get caught

The expensive mistake is spending the whole year's income and meeting the first January bill from overdraft or credit. The second most expensive is ignoring the July instalment because "the return isn't due yet" — it is a real payment deadline with interest attached.

Official sources: Payments on account (GOV.UK) · Self Assessment deadlines (GOV.UK)