You finished your first full tax year in reasonable shape. Profit of £40,000, you set aside what you thought was sensible, and you file in the second week of January feeling organised. The calculation comes back and asks for £11,900 by 31 January.
You had budgeted around £8,000. Nothing has gone wrong and HMRC has not made a mistake. You have met payments on account, and almost nobody sees them coming the first time.
The first bill contains two tax years
For a self-employed person, the tax year runs to 5 April. The online filing deadline and the payment deadline for that year are both 31 January following, so the year ending 5 April 2025 is filed and paid by 31 January 2026. Paper returns are due by the preceding 31 October.
On that same 31 January, HMRC also asks for the first payment on account towards the following tax year, equal to 50% of the liability just calculated. A second payment on account, the other 50%, falls due on 31 July. The following January you settle the difference between what you actually owed and what you paid on account, and simultaneously make the first payment on account for the year after that.
So the first January is one and a half years of tax. Every January after that is a balancing figure plus a half, which feels much more manageable because the halves are already in the system.
Roughly, on £40,000 of self-employed profit at current rates, you would expect income tax on the amount above the £12,570 personal allowance at 20%, plus Class 4 National Insurance on profits between £12,570 and £50,270. The personal allowance and the higher rate threshold have been frozen for several years, and the freeze has been extended more than once, so check the current position rather than assuming. Class 2 National Insurance ceased to be a required payment for most self-employed people from April 2024, though voluntary contributions remain available to protect a State Pension record for those with low profits.
Two payments on account of about £4,000 each, on top of an £8,000 balancing payment, is where the £11,900 comes from.
You can reduce them, and you should be careful about it
If you know your next year will be lower, you can apply to reduce your payments on account, either in the return itself or on form SA303. People whose trade has genuinely shrunk, or who have taken a salaried job, should do this.
The risk is that if you reduce them and the year turns out better than you expected, HMRC charges interest on the shortfall from the original due dates, as though you had underpaid all along. Reduce them on evidence, not on hope.
There are two circumstances where payments on account are not due at all: where your last Self Assessment bill was under £1,000, or where more than 80% of your tax was collected at source, for instance through PAYE on employment income.
Set money aside monthly, in a different account
The mechanism that works is the dullest one. Open a second business account. On the same day each month, move a fixed percentage of everything that came in.
For most sole traders trading below the higher rate threshold, 25% to 30% of net profit is a workable reserve for income tax and Class 4 National Insurance together. If you are approaching or above the higher rate threshold, push it to 40%. If you are VAT-registered, the VAT you collect belongs in that account too and is not a percentage of profit but a real liability calculated quarterly.
The reason to use a separate account rather than a spreadsheet is that a spreadsheet does not stop you spending the money. A reserve you have to deliberately transfer back is a reserve that survives a quiet February.
Keep the records properly as you go. HMRC requires you to keep business records for at least five years after the 31 January submission deadline for the relevant tax year. Making Tax Digital for Income Tax is being phased in for the self-employed and landlords by income level, starting with the highest, and it will require quarterly digital updates rather than one annual return. The thresholds and start dates have moved several times, so check where you fall before assuming it does not apply.
Penalties are automatic and interest is not the worst part
Miss the filing deadline and a £100 penalty applies immediately, even if you owe nothing. After three months, daily penalties of £10 a day can run for up to 90 days. At six and twelve months there are further penalties based on the tax due. Late payment carries its own penalties at 30 days, six months and twelve months, plus interest running from the due date.
A points-based penalty system has been introduced for Making Tax Digital filing obligations, replacing the automatic single penalty for those within it, so again, check which regime you are in.
The practical advice is unchanged in either case. If you cannot pay, file anyway, on time, and then ring HMRC. Filing and paying are separate obligations and separate penalties, and a return filed on time with a Time to Pay arrangement behind it is a much better position than silence.
HMRC operates an online Time to Pay service for Self Assessment debts below a threshold, and the arrangement typically spreads the balance over up to twelve months with interest. It is granted far more readily before the deadline than after it.
A few things that reduce the bill legitimately
Claim what you are entitled to and nothing you are not. Allowable expenses are those incurred wholly and exclusively for the trade. Use of home as an office can be claimed on a simplified flat rate by hours worked, or on an apportioned actual cost basis. Mileage can be claimed at the approved rates, historically 45p a mile for the first 10,000 business miles and 25p thereafter for cars, and those rates are set by HMRC and reviewed. Capital allowances, including the Annual Investment Allowance, cover equipment and vans.
Pension contributions reduce taxable income and are the largest lever most sole traders have. Charitable gifts under Gift Aid extend your basic rate band. None of this is exotic and all of it is easier if the records exist.
Where this shows up in the game
Business Baron puts the January bill on the calendar and does not remind you gently. Players who have taken money out of the business all year meet payments on account the same way real sole traders do, which is as a surprise with a deadline attached. Setting aside monthly is a habit the game rewards quietly rather than announcing. The first January arrives soon enough to matter, in the free game as much as the full one.