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Onshore vs offshore bonds: how top-slicing relief differs
The relief is computed the same way for both. The split lands in the net liability: an onshore bond carries a 20% basic-rate credit; an offshore bond carries none.
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Both bond types use the same five-step top-slicing computation (ITTOIA 2005 s.535–537; IPTM3820). The difference sits upstream of the relief and lands in the net liability: an onshore bond's life fund has already borne tax inside the fund, so the individual is treated as having paid basic-rate tax on the gain — a 20% credit. An offshore bond's fund rolls up gross, so there's no credit and the whole liability falls on the individual.
The credit, on a concrete gain
Take the offshore case the top-slicing articles use: a £60,000 gain over six complete years (slice £10,000), £35,000 of other income, 2025/26. Offshore, the gain carries no credit. An onshore bond with the same gain carries a basic-rate credit of 20% — £12,000 — treated as already paid.
- Gain
- £60,000
- Basic-rate credit — onshore (20%)
- £12,000
- Basic-rate credit — offshore
- £0
The credit is notional: treated as paid, never repayable. A non-taxpayer cannot reclaim it. The five relief steps then run identically on both wrappers — top-slicing reduces the tax on the slice the same way regardless of bond type. What changes is the final net tax: onshore, the individual deducts the £12,000 credit and pays only the excess over basic rate; offshore, there is nothing to deduct. The onshore net liability is therefore lower — by at most 20% of the gain, and never below zero.
The common error
Two wrappers, two mistakes. Applying an offshore method to an onshore bond drops the credit and overstates the tax; applying an onshore method to an offshore bond invents a credit that is not there and understates it. The relief is identical — the credit is the switch, and it is a function of the bond type on the certificate, not a choice. A subtler trap: the credit cannot turn a liability negative or generate a repayment, so a basic-rate or non-taxpayer client sees no onshore advantage on the gain itself.
The top-slicing relief calculator takes the bond type and applies the credit on the right side of the computation; compute the chargeable event gain first if you are working from the certificate figures.
ITTOIA 2005 s.530 (basic-rate credit) · IPTM3810 (onshore) / IPTM3210 (offshore gross roll-up) · IPTM3820 (top-slicing) — relief steps engine-computed ; onshore net figure to be grounded before publish
Related reading
Grounding & sources
- Offshore worked figures (£60,000 gain · 6 complete years · slice £10,000 · £35,000 other income, 2025/26) reuse IPTM-EX-03-OFFSHORE — the engine’s regression anchor (calc-engine/corpus/iptm-corpus.json), already published in the top-slicing articles.
- Onshore basic-rate credit = 20% of the gain, treated as paid (ITTOIA 2005 s.530; IPTM3810). The article states the credit (£12,000 on a £60,000 gain — rule-based arithmetic) but NOT a final onshore net-tax figure: that must be engine-computed (bond/net-liability.ts) and inserted before publish.
- Direction and cap of the onshore-vs-offshore difference follow the engine property invariant: 0 ≤ netLiability_offshore − netLiability_onshore ≤ gain × 20%, clamped at zero (CLAUDE.md; bond/__tests__/tsr-properties.test.ts).
- Competitor tools referenced by category only — no named vendor.
For planning and illustration purposes only. Verify all inputs against source documents. This explainer does not constitute financial or tax advice.