How IPTM3820 / HMRC Agent Update 83 (2021) changed top-slicing — and which tools haven't caught up

Top-slicing relief is the mechanism that stops a chargeable-event gain on a life-assurance bond being taxed as though the whole gain arose in a single year, when in substance it accrued across the life of the policy. The relief has a fixed five-step shape set out in HMRC's Insurance Policyholder Taxation Manual at IPTM3820, and the arithmetic of those steps has not changed. What changed in 2021 is a single input that flows through two of the steps: the Personal Savings Allowance.

For gains arising on or after 6 April 2021, IPTM3820 / HMRC Agent Update 83 (2021) confirmed that the Personal Savings Allowance (and, where relevant, the starting rate for savings band) must be recalculatedat the notional income level used inside the relief computation, rather than fixed once at the taxpayer's actual income and carried through. It sounds like a footnote. On the wrong side of a rate boundary it moves the relief by hundreds of pounds per bond.

The rule

Walk the five steps. Step 1 is the individual's total income-tax liability for the year, including the full gain. Step 2 strips out the relief that is being computed. Step 3 is the “total tax” figure: the liability on income includingthe full gain, with the gain treated as the top slice of income. Step 4 is the “tax on the annual equivalent”: the liability computed on income that includes only one year's slice of the gain (the gain divided by the number of complete years, N). The relief is, broadly, the difference between the tax attributable to the gain at Step 3 and the tax attributable to the sliced gain at Step 4, scaled back up by N.

The Personal Savings Allowance depends on which tax band the individual falls into: £1,000 for a basic-rate taxpayer, £500 for a higher-rate taxpayer, nil for an additional-rate taxpayer. Here is the pivot. At Step 3 the income figure includes the whole gain, which can push the taxpayer into higher rate, fixing the PSA at £500. At Step 4 the income figure includes only the sliced gain, which can leave the same taxpayer in basic rate, where the PSA is £1,000. IPTM3820 (HMRC Agent Update 83, 2021) says: use the band that applies at each step's own income level. Recalculate. Do not carry the Step-3 figure into Step 4.

A worked example

Take the case our calculation engine regression-tests against on every commit — an offshore bond, so there is no notional 20% credit to mask the effect. The figures below are byte-for-byte the engine's expected outputs (corpus row IPTM-EX-03-OFFSHORE), not illustrative round numbers.

An offshore bond is fully surrendered in 2025/26. The chargeable gain is £60,000, accrued over six complete years, so the annual equivalent (the slice) is £10,000. The individual has £35,000 of other income. With the full £60,000 gain stacked on top, total income is £95,000 — firmly higher rate, so the PSA at Step 3 is £500. With only the £10,000 slice stacked on top, the notional income is £45,000 — basic rate, so the PSA at Step 4 is £1,000.

The extra £500 of PSA at Step 4 widens the band of the slice that is taxed at 0%, worth £100 of tax a year. Scaled by N = 6, that is £600. Compute the same case the old way — carrying the Step-3 PSA of £500 into Step 4 — and the relieved liability rises to £11,400 and the relief falls to £9,446. Same bond, same gain, same other income; a £600 swing that comes entirely from which PSA you use at Step 4.

The offshore framing matters for seeing the effect cleanly. On an onshorebond the basic-rate slice carries a notional 20% credit, which in this particular case happens to absorb the PSA divergence so the final relief lands at the same number under both methods. That is exactly why the error hides: a tester who only checks an onshore case can carry the Step-3 PSA forward, see the “right” answer, and never notice the method is wrong. Move to offshore — or to a case where the credit does not fully absorb the gap — and the £600 appears.

The common error

The mistake is structural, not arithmetic. A spreadsheet built before April 2021 — or built afterwards by someone working from a pre-2021 worked example — computes the PSA once, from the taxpayer's banding with the full gain included, and then reuses that single figure at every step. It never asks the question IPTM3820 (HMRC Agent Update 83, 2021) forces: what band is the taxpayer in when only the slice is counted? For any case where the full gain crosses a rate boundary that the slice does not, the carried-forward PSA is too small at Step 4, the relieved liability is too high, and the relief is understated. The client pays more tax than they should.

It is a quiet error. It produces a plausible number. It will not trip a sense-check, because the relief is still positive and still in the right order of magnitude. The only way to catch it is to recalculate the allowance at each step — or to test against an HMRC-grounded case where the divergence is visible, which is the deferred/matched discipline our public corpus is built around.

ParaplanAI computes the PSA from scratch at each step — a pure (income, config) → allowance call invoked independently at Step 3 and Step 4, with no carry-over. The decision, and the reading of “recalculated” we apply, is documented in ADR-010; both step-level PSA figures are exposed on the compliance annex so the working is inspectable.

The citation

The five-step structure of the relief is at IPTM3820(“Calculating top slicing relief”) in HMRC's Insurance Policyholder Taxation Manual. The recalculation of the Personal Savings Allowance at the notional income level, for gains on or after 6 April 2021, is set out in IPTM3820 and confirmed in HMRC Agent Update 83 (April 2021). It sits alongside the reinstatement of the personal allowance in the top-slicing calculation under FA 2020 s.37, which followed the First-tier Tribunal decision in Silver v HMRC. The underlying allowance figures (£1,000 / £500 / nil by band) are in the savings-allowance provisions of ITA 2007.

IPTM3820 · HMRC Agent Update 83 (2021) · FA 2020 s.37 · ITA 2007 (personal savings allowance) · engine reading ADR-010

If you want to run the case yourself, the same offshore scenario is on the no-sign-up bond calculator, and the corpus row it is pinned to is published in full on the transparency page.