Bonus to ISA vs Pension: £5,000 Bonus Allocation

Quick answer

You've been awarded a £5,000 bonus. The bonus is taxed at your marginal rate through PAYE — so a higher-rate taxpayer keeps only £2,900 of it; an additional-rate taxpayer £2,650; someone caught in the £100k–£125,140 trap just £1,900.

You then have three rough options:

1. Pay the bonus into an ISA after tax — easy access, tax-free growth.
2. Salary sacrifice the bonus into your workplace pension before tax — much larger pot, locked until 55/57.
3. Hybrid: sacrifice part into pension and put part into an ISA.

For most higher-rate UK employees, pension wins on raw money — and by a lot. The only good reason to favour ISA is access before retirement.

How a £5,000 bonus actually flows

Before any of these decisions, the bonus goes through PAYE in the month it is paid. Bonus taxation feels brutal because the in-month deduction uses the cumulative tax allowance, but annually the bonus is taxed at the same rate as the rest of your year-end income — it does not get its own punitive rate.

For our worked examples, assume the bonus pushes you fully into the marginal band shown. See our bonus tax post for the in-month vs annual reconciliation mechanics.

Worked example 1 — basic-rate (£40,000 salary, £5,000 bonus)

Marginal rate: 28% (20% IT + 8% NI).

Option A — Bonus into ISA:

• Net bonus after tax: £5,000 × 72% = £3,600.
• ISA deposit: £3,600.
• 30-year growth at 5% real: £3,600 × 4.32 = £15,562.

Option B — Bonus salary-sacrificed into pension:

• Pension contribution: £5,000.
• Net pay reduction: £5,000 × 72% = £3,600 (same as the lost net bonus).
• 30-year growth at 5%: £5,000 × 4.32 = £21,610 in the pot.
• 25% tax-free at 55: £5,403 tax-free, £16,207 taxable.
• Pension taxed at basic rate in retirement (assuming basic-rate then too): £16,207 × 80% = £12,966.
• Total net: £5,403 + £12,966 = £18,369.

Pension beats ISA by £2,807 at basic rate, despite the same cost of giving up the bonus.

If the employer adds their 15% NI saving (£750) into the pension, total becomes £5,750 sacrificed → £24,851 at 30 years → about £21,124 net after PA-aware tax. Difference grows to £5,562.

Worked example 2 — higher-rate (£70,000 salary, £5,000 bonus)

Marginal rate: 42% (40% IT + 2% NI).

Option A — ISA:

• Net bonus: £5,000 × 58% = £2,900.
• 30-year growth: £2,900 × 4.32 = £12,533 tax-free.

Option B — Pension sacrifice:

• Pension contribution: £5,000.
• Net pay reduction: £2,900.
• 30-year growth: £21,610.
• 25% tax-free: £5,403.
• 75% taxable: if drawn within the basic-rate band in retirement, £16,207 × 80% = £12,966.
• Total net: £18,369.

Pension beats ISA by £5,836 for the same net cost.

If employer passes the 15% NI saving into the pension, the gap rises to roughly £8,000.

The arithmetic is unforgiving: the only situation where ISA beats pension at higher-rate marginal is if you expect to draw the pension at a higher marginal rate than you contributed it at. For most workers that is implausible.

Worked example 3 — £100k trap (£110,000 salary, £5,000 bonus)

Marginal rate: ~62% (40% IT + 2% NI + 20% PA withdrawal).

Option A — ISA:

• Net bonus: £5,000 × 38% = £1,900.
• 30-year growth: £1,900 × 4.32 = £8,208 tax-free.

Option B — Pension sacrifice:

• Pension contribution: £5,000.
• Net pay reduction: £1,900 (you avoid the 62% marginal).
• 30-year growth: £21,610.
• Net post-tax in retirement (assuming basic-rate): £18,369.

Pension beats ISA by £10,161 — more than double the wrapper return for the same cash.

This is why the bonus sacrifice into pension 60% tax trap post is one of the highest-leverage moves in UK personal finance. If you are between £100,000 and £125,140 of adjusted net income, do not let a bonus push you further into the trap — sacrifice the whole thing.

The LISA wildcard

If you are under 40 and either saving for a first home (under £450,000) or for retirement, the Lifetime ISA adds a different angle.

• LISA cap: £4,000 per year, with a 25% government bonus = £1,000 free.
• Net bonus after tax (higher-rate): £2,900.
• Put £2,900 into LISA, government adds £725 → £3,625 in the wrapper.
• Tax-free for life if used for a qualifying purpose.

A higher-rate earner sacrificing all £5,000 into pension still wins on raw maths (£18,369 net vs £15,660 LISA value at 30 years). But for a first-time buyer needing the deposit at age 35, LISA is the only option of the three because pension funds are locked.

See our LISA closure post for the cost of non-qualifying withdrawal.

The hybrid 60/40 strategy

For most higher-rate professionals with no immediate cash need, a sensible default is:

• 60% into pension — captures the marginal-rate uplift.
• 40% into ISA — keeps a pot accessible before 55.

A £5,000 bonus split this way:

• Sacrifice £3,000 into pension; net pay reduction £1,740.
• Receive £2,000 of bonus net, after PAYE = £1,160 in hand.
• Deposit £1,160 into ISA.

Total pension pot increase: £3,000 (or £3,450 with employer NI passed on).
Total ISA: £1,160.
Net pay reduction: £2,900.

Same upfront cost as a full pension sacrifice, but you retain pre-55 liquidity for emergencies.

What if I am over 55?

Once you reach normal minimum pension age (55, rising to 57 in 2028) the access argument disappears. A bonus sacrifice into pension can be partly drawn back out within months — the 25% tax-free lump sum on a £5,000 contribution is £1,250, available immediately.

This makes near-retirement pension sacrifice particularly powerful — but watch the Money Purchase Annual Allowance (MPAA). Drawing taxable pension income triggers a £10,000 reduced annual allowance, which makes future contributions harder.

When ISA wins

A small number of cases:

1. You are confident you will be a higher-rate taxpayer in retirement (rare).
2. You need access to the money before 55/57 (very common in your 20s and 30s).
3. You are at the lifetime pension limit (post-LTA-abolition this only matters for the LSDBA at £1,073,100).
4. You distrust future pension tax rule changes — a real consideration, although ISAs are not immune either.

For most working-age UK higher-rate taxpayers, the answer is: sacrifice the bonus, fill the ISA from other savings.

Sources

• HMRC: Tax on your private pension contributions
• HMRC: Individual Savings Accounts (ISAs)
• gov.uk: Lifetime ISA
• HMRC: Income Tax rates and Personal Allowance