Retirement: Pensions, 401(k)s, and SIPPs — making sense of retirement account types

The same investment, held in a more tax-efficient account type, can be worth materially more at retirement. Account choice is not bureaucracy. It is strategy.

Retirement account types — pensions, ISAs, 401(k)s, SIPPs, IRAs, and their variants — are often treated as administrative necessities rather than strategic tools. This is a missed opportunity. The tax treatment of these accounts varies significantly, and the difference between using them well and using them poorly compounds over decades.

There is also a framing issue worth addressing. These accounts are called "retirement accounts" — and for many people, that label is either discouraging ("retirement is so far away") or feels irrelevant ("I might never be able to afford to retire"). A more useful way to think about them, particularly for anyone planning a non-linear working life, is as time-restricted investment accounts: wrappers that offer generous tax advantages in exchange for keeping money invested for a defined minimum period.

For someone planning 100 great years — who might want to stop traditional employment at 50, pivot to lower-paid but more meaningful work, take a sabbatical, or move in and out of employment — having a sequence of accessible pools of money becomes critical. A Stocks & Shares ISA is fully accessible at any time; a Roth IRA allows contributions (though not earnings) to be withdrawn without penalty at any age. A SIPP or 401(k) is accessible from a specific age. A taxable brokerage account is accessible from day one. Building all three, in the right order, creates a ladder of accessible wealth rather than a single pot that opens at 65. That ladder is what financial flexibility across a long life actually looks like.

The core principle: tax-advantaged wrappers

All retirement account types share one characteristic: they provide some form of tax advantage compared to holding the same investments in a standard taxable account. The tax benefit takes one of two forms:

Tax relief on contributions (EET — Exempt, Exempt, Taxed): Contributions are made from pre-tax income, reducing your tax bill now. The investment grows tax-free inside the wrapper. Withdrawals are taxed as income. UK pensions, US 401(k)s and traditional IRAs, and US traditional 403(b)s follow this structure. Effective for those who expect to be in a lower tax bracket in retirement than during their working years.

Tax-free growth and withdrawals (TEE — Taxed, Exempt, Exempt): Contributions are made from after-tax income — no upfront tax relief. Investment growth is tax-free and withdrawals are tax-free. UK ISAs and US Roth IRAs and Roth 401(k)s follow this structure. Effective for those who expect the same or higher tax rates in retirement, or who want maximum flexibility in withdrawal.

Both structures provide significant advantages over taxable accounts, where investment returns are subject to capital gains tax, dividend tax, and income tax on interest. Over long periods, the tax drag on a standard account can materially reduce final wealth compared to either tax-advantaged structure.

UK: the main account types

Workplace pension (auto-enrolment scheme): Most UK employees are automatically enrolled in a workplace pension. The employer contributes a minimum of 3% of qualifying earnings; the employee contributes a minimum of 5%, of which 1% comes from government tax relief. Employer contributions are free money — always capture the full employer match before making any other savings decision.

Pension contributions receive tax relief at your marginal rate. A basic-rate taxpayer contributing £80 receives £100 invested (£20 government top-up). A higher-rate taxpayer claiming relief through self-assessment receives effectively £60 net cost for £100 invested.¹ The annual pension allowance for 2026/27 is £60,000 (or 100% of earnings, whichever is lower) — this limit covers all pension contributions combined, including workplace pension and any personal SIPP contributions.

Self-Invested Personal Pension (SIPP): A personal pension with full investment flexibility — you choose the investments held within it. Particularly useful for self-employed people without access to a workplace scheme, or for consolidating multiple old workplace pensions. The same tax relief rules apply as for workplace pensions, and contributions count toward the shared £60,000 annual pension allowance.

Stocks & Shares ISA: Annual allowance of £20,000 (2025/26). Contributions from after-tax income; all growth and withdrawals permanently tax-free. No minimum age for access (unlike pensions). The key vehicle for building accessible wealth before pension age — ideal for anyone who may want financial flexibility before 57, for those who anticipate tax rates rising, or as the "accessible" rung in a multi-account wealth ladder.

Cash ISA: Tax-free savings account. The same £20,000 annual allowance as Stocks & Shares ISA is shared across all ISA types. Appropriate for near-term savings needs; not a long-term wealth-building vehicle compared to an invested S&S ISA.

Priority order for most UK savers: (1) Employer pension to capture full match, (2) S&S ISA to build accessible, tax-free wealth before pension age, (3) Additional pension contributions (workplace or SIPP) toward the £60,000 annual allowance, (4) Taxable brokerage account once ISA and pension allowances are optimised.

US: the main account types

401(k) / 403(b) / TSP: Employer-sponsored defined contribution plans. The 401(k) is the most common private-sector variant; the 403(b) is used by non-profits, schools, and hospitals; the Thrift Savings Plan (TSP) is the federal government equivalent, offering the same tax structure with very low-cost index fund options. Contributions are pre-tax (reducing current taxable income); growth is tax-deferred; withdrawals in retirement are taxed as ordinary income. 2025 contribution limit: $23,500 ($31,000 for those 50+). Always contribute enough to capture the full employer match — this is the single highest-return financial decision available to most workers.²

Traditional IRA: Individual Retirement Account with the same EET tax structure as a 401(k). 2025 contribution limit: $7,000 ($8,000 for those 50+). Tax deductibility phases out at higher income levels for those also covered by a workplace plan.

Roth IRA: After-tax contributions; qualified withdrawals are tax-free. Contributions (not earnings) can be withdrawn at any time without tax or penalty. Earnings are subject to both an age rule (must be 59½) and a five-year rule (the account must have been open for at least five years) before they can be withdrawn tax and penalty-free. 2025 contribution limit: $7,000 ($8,000 for 50+). Income limits apply to direct contributions ($150,000+ for single filers, $236,000+ for married filing jointly in 2025). The "backdoor Roth" allows high earners to contribute via a non-deductible traditional IRA conversion.

Roth 401(k): Many employers now offer a Roth option within the 401(k) — same contribution limits as traditional 401(k) but with Roth tax treatment. Particularly valuable for younger workers in lower current tax brackets who expect to earn more in future, or for those who want to diversify their tax exposure in retirement.

Priority order for most US savers: (1) 401(k) to employer match, (2) Max HSA if on a high-deductible health plan (triple tax advantage), (3) Roth IRA to the limit, (4) Max 401(k) remainder, (5) Taxable brokerage account.

The account type question that matters most

For most people, the choice between Roth/ISA (after-tax contributions) and traditional pension/401(k) (pre-tax contributions) comes down to one question: will your marginal tax rate be higher now, or in retirement?

If you are in a lower tax bracket now than you expect to be in retirement (younger, early career, temporarily lower income year): favour Roth/ISA.

If you are in a higher tax bracket now than you expect in retirement (peak earning years, expecting pension income to be lower): favour traditional pension/401(k).

If uncertain, diversifying across both structures provides flexibility — you will have both taxable and tax-free income sources in retirement, giving you options to manage tax liability.

A note for higher-rate taxpayers in both the UK and US: if you are currently paying 40–45% (UK) or 32–37% (US) marginal tax and expect to pay a lower rate in retirement, the maths strongly favour pre-tax pension or 401(k) contributions over ISA or Roth contributions for money you will not need before pension access age. In the UK, a higher-rate taxpayer gets pension relief at 40%, meaning every £1 invested costs as little as 60p net. In the US, a 37% bracket taxpayer making pre-tax 401(k) contributions receives an effective 37% discount on each dollar invested. This is a structural advantage that compounds for decades and is not available in post-tax wrappers. The ISA and Roth remain valuable for accessible, flexible savings — the pension and 401(k) are superior for long-term locked-in wealth when marginal rates are high today.

The 100 Great Years perspective

The difference between using retirement accounts well and using them poorly is not one of investment skill — it is one of structure. Two people making identical investment choices but using different account types will arrive at retirement with materially different outcomes. 100 Great Years does not tell you which specific products to choose — that is personalised advice requiring a qualified adviser — but it does encourage every user to understand the tax wrappers available to them and to use them in the right order. The employer match, the tax relief, the compounding inside a sheltered wrapper: these are guaranteed advantages. Not using them is not a neutral choice.

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This article is for educational purposes only and does not constitute financial advice. Past performance is not a reliable indicator of future results. Always consider your personal circumstances and consult a qualified financial adviser before making investment decisions.