Why Is My First Self Assessment Tax Bill So High?

Payments on account are advance payments towards your next Self Assessment tax bill. Each one is normally half of the previous year's liability — the Income Tax and Class 4 National Insurance that was due through Self Assessment.

They exist because employees have tax collected from every payslip through PAYE, while self-employed people would otherwise pay a whole year's tax many months after earning it. Payments on account move part of that collection forward.

They are due on 31 January and 31 July. Class 2 National Insurance and Capital Gains Tax are not included in payments on account.

Because your first January payment covers a completed year in full and starts the next year at the same time. Take a first tax year with a £3,000 liability:

The extra £1,500 in January is not additional tax on the completed year. The completed year is fully settled by the £3,000 balancing payment. The £1,500 is an instalment towards the tax year you are in now, and it will be deducted from that year's liability when you file the next return.

Across the two dates you pay £3,000 for the finished year and £3,000 in advance for the current one — which is why the first year feels front-loaded and later years usually do not.

No. They are the same tax, collected earlier. When your next return is filed, the payments on account you have made are set against the liability for that year. If they were more than the tax due, the difference is refunded or set against what comes next; if they were less, you pay the balance.

According to current GOV.UK guidance, payments on account are not usually required if:

• the amount you owe through Self Assessment for the year is less than £1,000, or
• 80% or more of the tax you owe for the year has already been collected at source — most commonly through PAYE on a salary, or tax already deducted from other income.

The 80% test is why people with a large salary and a small side business often never see payments on account: nearly all their tax has already been taken through PAYE. It is judged on the tax you owe for the year, not on how your income is split.

The Payment on Account Calculator applies both tests to your figures and tells you which one applies.

Payments on account are based on last year's liability, so they can be too high if your trade has slowed, you have stopped self-employment, or you have moved to mainly employed work. HMRC allows you to claim to reduce your payments on account where you reasonably expect the next liability to be lower.

The caution is important: if you reduce them too far and the eventual bill turns out higher, HMRC can charge interest on the amount that should have been paid on the original dates. So a reduction should be based on a realistic estimate of the year, not on optimism or cash-flow pressure.

If cash flow is the real problem, GOV.UK also sets out how to arrange time to pay rather than under-declaring an estimate.

Two steps usually give a good picture:

1. Estimate the tax and Class 4 National Insurance for the year you are filing with the Self-Employed Tax Calculator UK.
2. Feed that liability into the Payment on Account Calculator to see the balancing payment and each instalment, with their dates.

If you would rather build the money up gradually, the How Much Should I Save for Tax? calculator converts the same figures into a monthly amount to set aside.