Every year a chunk of UK Self Assessment filers open the 31 January HMRC bill and ask the same question: “Why is this 50% larger than the tax I actually owe?” The answer is the balancing payment plus first POA stack — a specific quirk of how HMRC interleaves prior-year and current-year tax collection. This article maps out exactly what each line on the SA300 statement is, walks through the 50/50 + balance arithmetic, and shows three worked scenarios (rising income, falling income, first POA year) so you can predict next January’s bill before HMRC sends it.
The Two Payment Types in One Sentence
- Balancing payment = what you actually owed for the prior tax year, minus what you already paid as POAs during that year. Settled by 31 January following the tax year.
- Payment on Account (POA) = an advance instalment towards the current tax year, set at 50% of the prior year’s liability, paid in two halves (31 January and 31 July).
Each January 31 bundles both: last year’s balance + this year’s first POA.
What’s on Your January 31 Bill
For the 31 January 2027 deadline (covering 2025-26 Self Assessment), HMRC’s SA300 statement typically shows:
| Line | What it is | Calculation |
|---|---|---|
| Balancing payment 2025-26 | Actual 2025-26 SA liability − POAs already paid in Jan/Jul 2026 | Variable |
| First POA for 2026-27 | 50% of 2025-26 liability | Half of last year’s actual |
| Class 2 NI 2025-26 | Flat annual amount (currently £3.45 / week × 52) | ~£179 |
| Capital Gains Tax 2025-26 | Single payment, no POAs apply | Variable |
| Student loan 2025-26 | Single payment, no POAs apply | Variable |
| Total due 31 January 2027 | Sum of the above | The number that hurts |
The next 31 July 2027 then bills only the second POA for 2026-27 (50% of 2025-26 liability) — that’s why summer feels lighter. Then 31 January 2028 starts the cycle again with the 2026-27 balancing payment + first 2027-28 POA.
The 50/50 + Balance Maths in One Diagram
For tax year N with actual liability £X, the cash flow looks like this:
31 Jan (year N): POA 1 for year N = 50% of (year N-1 liability)
31 Jul (year N): POA 2 for year N = 50% of (year N-1 liability)
─────────────────────────────────────────────────
Total advance paid = 100% of (year N-1 liability)
31 Jan (year N+1): Balancing payment = X − [100% of (year N-1)]
+ POA 1 for year N+1 = 50% of XThe balancing payment is positive (you owe more) if your liability went up, negative (refund) if it went down.
Worked Example A — Rising Income (most common painful case)
Olaria is a freelance consultant. 2024-25 SA liability: £8,000. 2025-26 turns out to be £12,000 (more clients, big year-end project).
Cash flow:
| Date | Item | Amount | Running balance owed/(refunded) |
|---|---|---|---|
| 31 Jan 2026 | POA 1 for 2025-26 (50% × £8,000) | £4,000 | – |
| 31 Jul 2026 | POA 2 for 2025-26 (50% × £8,000) | £4,000 | – |
| POAs paid towards 2025-26 | £8,000 | – | |
| Actual 2025-26 liability | £12,000 | – | |
| 31 Jan 2027 | Balancing payment (£12,000 − £8,000) | £4,000 | – |
| 31 Jan 2027 | POA 1 for 2026-27 (50% × £12,000) | £6,000 | – |
| 31 Jan 2027 | Total due | £10,000 | – |
Olaria’s 31 Jan 2027 bill is £10,000 — 25% more than her actual 2025-26 tax of £12,000 minus what she already paid (£4,000 balancing) — because of the additional £6,000 first POA stack.
She also owes £6,000 on 31 Jul 2027 (POA 2 for 2026-27). Then 31 Jan 2028 will bring the 2026-27 balancing payment + 2027-28 POA 1.
Worked Example B — Falling Income (refund, smaller January bill)
Marcus, same baseline: 2024-25 SA liability £8,000. But 2025-26 turns out to be £4,000 (took a sabbatical, low income).
Cash flow:
| Date | Item | Amount |
|---|---|---|
| 31 Jan 2026 | POA 1 for 2025-26 (50% × £8,000) | £4,000 |
| 31 Jul 2026 | POA 2 for 2025-26 (50% × £8,000) | £4,000 |
| POAs paid | £8,000 | |
| Actual 2025-26 liability | £4,000 | |
| 31 Jan 2027 | Balancing refund (£4,000 − £8,000) | (£4,000) |
| 31 Jan 2027 | POA 1 for 2026-27 (50% × £4,000) | £2,000 |
| 31 Jan 2027 | Net position | (£2,000) refund |
Marcus is owed £2,000 on 31 Jan 2027 — HMRC pays him £2,000 net (£4,000 refund minus £2,000 next POA owed). He could have applied to reduce his 2025-26 POAs in advance via SA303 if he’d known his income would drop, freeing up the £8,000 cash flow earlier in 2026 — see the POA reduction guide.
His 31 Jul 2027 POA 2 is £2,000.
Worked Example C — First-Year POA Shock
Priya crossed the £1,000 SA threshold for the first time. 2023-24 SA was £900 (no POAs triggered). 2024-25 jumped to £5,000 as her freelance side-hustle grew.
Cash flow:
| Date | Item | Amount |
|---|---|---|
| 31 Jan 2025 | POA 1 for 2024-25 — none (2023-24 was below threshold) | £0 |
| 31 Jul 2025 | POA 2 for 2024-25 — none | £0 |
| POAs paid towards 2024-25 | £0 | |
| Actual 2024-25 liability | £5,000 | |
| 31 Jan 2026 | Balancing payment (£5,000 − £0) | £5,000 |
| 31 Jan 2026 | POA 1 for 2025-26 (50% × £5,000) | £2,500 |
| 31 Jan 2026 | Total due | £7,500 |
Priya’s January 2026 bill is £7,500 — 50% more than her actual £5,000 liability, with another £2,500 due 31 July 2026. This is the single most painful Self Assessment moment for newly-self-employed people. Plan ahead: from the moment your prior-year liability crosses £1,000, set aside 130-150% of expected current-year tax in a dedicated savings account.
Why HMRC Doesn’t Just “Bill the Difference”
The system was designed in 1996 (Tax Modernisation) to smooth HMRC’s own cash flow, not to make taxpayers’ lives easy. By the time you file your 2025-26 return (deadline 31 January 2027), HMRC has already had to wait nearly 22 months from 6 April 2025. The two POAs let them collect 100% of last year’s liability across the current year — close to a real-time collection model.
For taxpayers it shifts the cash demand from “1 big bill on 31 Jan, 22 months after the tax year started” to “2 medium bills (31 Jan + 31 Jul) advance + 1 reconciliation Jan after.” Counter-intuitively, this is friendlier to people with stable income (who barely notice the smoothing) and harsher on people in their first POA year or with rapidly rising income.
How to Plan for the January Stack
Three rules of thumb that work for most Self Assessment filers:
-
Save 130-150% of expected current-year SA tax in a dedicated savings account during the year. The buffer covers the balancing payment + first POA stack.
-
Reduce POAs proactively if you can prove income will drop, but never reduce to zero unless you’re certain (HMRC charges interest on shortfall from the original POA due dates, not from 31 January). Reduction guide.
-
Use Time to Pay if cash flow is genuinely tight — HMRC offers payment plans up to 12 months for Self Assessment debts under £30,000 via your Personal Tax Account, no negotiation required. Interest accrues at the late-payment rate (~8.5%) but no penalty applies as long as the plan is set up before 31 January.
Frequently Asked Questions
Q: My SA300 says I owe £15,000 but my actual 2025-26 tax was £8,000. How? The most common cause: balancing payment for 2024-25 + first POA for 2025-26 stack. If 2024-25 was £14,000 and you paid £7,000 of POAs during 2025, your balancing payment is £7,000 + first POA for 2025-26 (£8,000 × 50%) = £4,000. Total £11,000. If the SA300 shows £15,000, you might also have Class 2 NI (~£180) and CGT or student loan added in. Check the line items.
Q: Can I just pay the balancing payment and skip the POA? No — both are statutory due dates under TMA 1970 s 59A and s 59B. Skipping the POA accrues HMRC interest from 1 February. There’s no penalty, but interest at ~8.5% adds up over 6 months until the next deadline catches you.
Q: I was an employee for 2025-26 — do I still owe a POA based on 2024-25 self-employment? Yes, unless you applied to reduce the 2025-26 POAs to zero before they fell due. HMRC bills the POAs based on prior-year liability regardless of current-year circumstances. File a reduction claim (SA303) as soon as you know your circumstances changed.
Q: My income is highly seasonal — half in Q4, nothing in Q1. Does HMRC reduce my POAs? No, the 50/50 split is fixed regardless of when income is earned. The annualised income method that exists for US estimated tax does not have a UK equivalent. You can only reduce POAs based on annual lower-income forecasts, not seasonality.
Q: Are POAs treated as paid evenly across the year for interest calculations? For Self Assessment interest purposes, each POA is dated on its actual payment date. There’s no equivalent of the US “withholding deemed paid evenly” rule. A POA paid on 30 July counts as one day late (interest on that day’s worth), not as having shifted earlier.
Q: My partner and I both file SA — can we combine POAs? No. The UK doesn’t have joint Self Assessment filing. Each individual’s POA is calculated on their own prior-year liability. Marriage Allowance can transfer £1,260 of personal allowance between spouses but does not merge POA obligations.
Sources
- HMRC: Self Assessment payment deadlines
- HMRC: Pay your Self Assessment tax bill — payments on account
- HMRC: Time to Pay arrangements
- TMA 1970, s 59A (payments on account), s 59B (balancing payment)
Predict your January bill before HMRC sends it
Use the payment on account calculator to project your January 2027 stack from your 2024-25 liability and 2025-26 estimate. If your income is dropping, the POA reduction guide walks through the SA303 process. If you missed a deadline, the HMRC late-payment interest article shows what’s accruing day by day.