Payment on account (POA) is the part of Self Assessment that catches the most self-employed dentists off guard. Instead of paying your tax once a year after you file, HMRC asks you to pay it in advance, in two instalments, based on what you owed the year before. For an associate who has just left a salaried or foundation role, the timing can feel brutal until you understand the mechanics.
This guide walks through how POA works for a self-employed dentist: the two dates, the 50% calculation, the thresholds that decide whether you are in the system at all, the first-year double hit, and how to reduce payments safely when your income falls. It sits alongside our wider associate dentist tax guide and the practical step-by-step Self Assessment filing guide.
What payment on account actually is
Payment on account is HMRC's way of collecting tax from the self-employed closer to when the income is earned. The legal basis is section 59A of the Taxes Management Act 1970. The system makes you pay, in advance, towards the current tax year's bill, using last year's liability as the estimate.
It covers income tax and Class 4 National Insurance only. It does not include capital gains tax, and it does not include Class 2 National Insurance, which was abolished from 6 April 2024 for the self-employed. Any capital gains tax you owe is settled separately as part of your balancing payment on 31 January, not spread across the two interim payments.
Are you even in the system? The two thresholds
You are only asked to make payments on account if both of these are true for the year you have just filed:
- Your Self Assessment tax bill was more than £1,000, and
- Less than 80% of your tax for the year was collected at source (for example through PAYE on employment, or tax deducted before you received the income).
If your bill was £1,000 or under, no POA. If 80% or more of your tax was already deducted at source, no POA. This is why a foundation dentist still on a salaried PAYE contract, or an associate with only a small amount of private work on top of an employed hospital role, may never enter the system. Once you move to full self-employed associate work, both thresholds are usually crossed quickly.
The two dates and the 50% calculation
There are two interim payments each year, and each one is 50% of your previous year's income tax plus Class 4 NIC liability:
| Date | What you pay |
|---|---|
| 31 January | First payment on account for the current year (50% of last year's liability), due alongside any balancing payment for the year just filed |
| 31 July | Second payment on account for the current year (the other 50%) |
HMRC calculates the figures automatically from your filed return, but you can sanity-check them yourself:
- Take your income tax for the year just filed
- Add your Class 4 National Insurance for that year
- Ignore any capital gains tax and Class 2 NIC, which are not part of POA
- Divide the result by two to get each payment on account
Worked illustration: suppose your income tax plus Class 4 NIC for the year came to £12,000. Each payment on account is £6,000, due on 31 January and 31 July. Together they pre-fund £12,000 towards the next year's bill. When you eventually file that next year, anything owed above £12,000 becomes a balancing payment, and anything below is set against your next instalment or refunded.
The first-year double hit
The single most uncomfortable feature of POA arrives in your first year inside the system. On that first 31 January you pay two things at once:
- The balancing payment for the tax year you have just filed (the full bill for that year), and
- The first payment on account for the current year (50% of that same bill again).
So you settle 100% of one year and pre-pay 50% of the next, on the same day. That is the first-year double hit, and it means your first January bill is effectively 150% of a normal year's tax. Plan cash flow for it well ahead of time, because it lands at the same moment you may also be settling personal-tax and pension figures.
Year 1 versus year 2 cash flow
The example below follows an associate whose income tax plus Class 4 NIC settles at around £20,000 a year once self-employment is in full swing. Figures are rounded for illustration.
| When | Payment | Amount |
|---|---|---|
| 31 Jan (year 1, first filing) | Balancing payment for the year filed | £20,000 |
| 31 Jan (year 1, same day) | First payment on account for current year | £10,000 |
| Total on that first 31 January | Double hit | £30,000 |
| 31 Jul (year 1) | Second payment on account | £10,000 |
| 31 Jan (year 2) | Balancing payment (if liability unchanged, this is nil) plus first POA for next year | £10,000 |
| 31 Jul (year 2) | Second payment on account | £10,000 |
The pattern is clear. The first January is heavy at £30,000 because you are paying a full year and pre-paying half of the next. From year two onwards, if your income holds steady, the rhythm settles into two payments of £10,000 each year and the balancing payment falls to nil. The shock is front-loaded into that first filing, so the work is to reserve for it before it arrives.
What happens when income rises or falls
Because POA is built on last year's figures, it always lags reality. That matters in both directions.
When your income rises, your two payments on account are set from a lower prior year, so they under-fund the bigger bill coming. The gap shows up as a larger balancing payment the following 31 January, and your payments on account then step up too. Rising associates should keep reserving above the headline POA amounts so the catch-up does not bite.
When your income falls, the opposite problem appears. Your payments on account are set from a higher prior year, so you are over-paying towards a smaller bill. You do not have to simply hand HMRC the cash and wait for a refund. You can apply to reduce the payments instead.
Reducing payments on account, and the interest risk
If you genuinely expect a lower liability this year, you can apply to reduce both payments on account. You do this either through your online Self Assessment account or on form SA303. Common reasons for a legitimate reduction include:
- A drop in associate days or sessions, or a planned career break
- Maternity, paternity or extended leave reducing the year's income
- A move from self-employed associate work back into a salaried PAYE role
- A material change in income structure, such as buying in or selling out of a practice
The discipline is to keep the estimate realistic. If you over-reduce, meaning you reduce your payments below what you actually end up owing, HMRC charges interest on the shortfall, calculated from the original due dates rather than from when you eventually pay. Reducing because you have an evidenced expectation of lower income is sensible. Reducing simply to ease short-term cash flow, with no real basis, is how associates end up with an interest bill on top of the tax.
If your income is changing because of a practice transition, apply for the reduction as soon as the new income level is reasonably clear, and back it with projections rather than guesswork. A formal, evidenced reduction protects you. Quietly underpaying without a claim does not, and it triggers automatic interest.
Common payment on account mistakes
Forgetting the July payment. The 31 July instalment is easy to overlook because it arrives mid-year with no filing attached to it. Diarise it the moment you settle January.
Treating the first January bill as a one-off error. It is not a mistake. The double hit is how the system always behaves in your first year, so do not assume HMRC has overcharged you.
Over-reducing to fix cash flow. Cutting payments without a genuine expectation of lower income converts a timing problem into an interest charge.
Not reserving monthly. The single most effective habit is moving a fixed share of each month's net income into a separate tax account, so the January and July dates are funded long before they arrive.
Planning for it from day one
The associates who handle payment on account well are the ones who treat it as a known event rather than a surprise. From your first month of self-employed associate income, set aside a consistent share of net income into a dedicated tax savings account. That reserve covers the first-year double hit and then funds the ongoing twice-yearly rhythm without disruption.
If you are moving from a foundation or hospital PAYE role into self-employed associate work, the jump in administration is the part most people underestimate. Get your first full year's figures modelled early, because by the time the first 31 January arrives, the payments on account have already been calculated and the timing is fixed. For the wider picture on deadlines, expenses and reliefs, read our associate dentist tax guide, and for the mechanics of filing the return itself, see the step-by-step Self Assessment guide.