ISA vs SIPP: tax-free later or tax relief today

Basics10 min read · updated 3 November 2026 · 16 sources

Choosing between an ISA vs SIPP is the main decision for anyone in the UK who invests for the long run. A stocks and shares ISA takes money you have already paid tax on and lets it grow and come out tax-free. A self-invested personal pension (SIPP) adds tax relief on the way in and taxes most of what comes out. Which one leaves more depends on your tax rate today, your tax rate in retirement and how soon you may need the money. This article uses the rules for the 2026/27 tax year as of November 3, 2026, including the Budget of October 28, 2026.

How an ISA and a SIPP are taxed

Both wrappers shelter investments from tax on dividends, interest and capital gains while the money stays inside. The difference is when tax is paid. The Institute for Fiscal Studies describes ISAs as broadly taxed up front: contributions come from taxed income, while growth and withdrawals are free of tax. Pensions work the other way round: contributions get income tax relief, growth is tax-free and withdrawals are taxed as income, apart from the tax-free lump sum (Adam et al. 2023).

In 2026/27 you can put up to £20,000 into ISAs, split as you like between cash, stocks and shares and the other types (HMRC, ISAs 2026). The plan from the Budget 2025 to limit cash ISAs to £12,000 a year for savers under 65 from April 2027 has been dropped: the Budget of October 28, 2026 keeps the cash ISA limit at £20,000 (Budget 2026 2026).

In a SIPP, you pay in from your net pay and the provider claims basic-rate relief of 20% from HMRC and adds it to your pot, so a payment of £8,000 becomes £10,000. Higher-rate and additional-rate taxpayers claim the rest of their relief through Self Assessment (HMRC, pension tax relief 2026). Relief is limited to 100% of your relevant UK earnings, or £3,600 gross if that is higher (HMRC, pension schemes rates 2026). The annual allowance for all pension contributions, including your employer's, is £60,000; it tapers away for adjusted income above £260,000, and unused allowance from the previous 3 tax years can be carried forward (HMRC, annual allowance 2026).

At retirement, you can usually take up to 25% of a pension as a tax-free lump sum, capped by a lump sum allowance of £268,275 over your lifetime; the rest is taxed as income in the year you withdraw it (HMRC, tax-free pension 2026).

Which one leaves more after tax

A fair comparison starts from the same amount of take-home pay. Suppose a basic-rate taxpayer can spare £8,000 of net income, invests it once and leaves it for 25 years at 5% a year. In the ISA, the £8,000 grows as it is. In the SIPP, relief at source turns it into £10,000 before it is invested.

For a basic-rate taxpayer, the break-even tax rate in retirement is 26.7%: the SIPP wins as long as withdrawals are taxed at 20% and loses if they would be taxed at 40%. For a higher-rate taxpayer, the break-even is 53.3%, above the top rate of 45%, so the pension wins on tax alone even for someone who pays the additional rate in retirement.

In England, Wales and Northern Ireland, income above the personal allowance of £12,570 is taxed at 20% up to £50,270, at 40% up to £125,140 and at 45% above (HMRC, income tax rates 2026). These thresholds are frozen until April 2031 (Finance Act 2026 2026). Scotland has six rates of its own, from 19% to 48% (HMRC, Scottish income tax 2026). Employer contributions tilt the comparison further toward the pension: money your employer pays into a workplace pension never reaches an ISA, so any employer match comes before both.

Access and flexibility

The ISA's advantage is access. You can withdraw from a stocks and shares ISA at any time without tax. A SIPP is locked until the normal minimum pension age, which is 55 today and rises to 57 from April 6, 2028 (HMRC, PTM062100 2026). For anyone planning to stop work early, the ISA is the bridge between leaving work and the pension age.

Once you take taxable money flexibly from a pension, the money purchase annual allowance cuts future contributions to £10,000 a year (HMRC, pension schemes rates 2026). Taking only the tax-free lump sum does not trigger it. Spreading pension withdrawals over several tax years uses the personal allowance and the basic-rate band every year; a large withdrawal in a single year can push part of it into the 40% band and undo the advantage shown above. How much to draw each year is the question of the safe withdrawal rate.

The Lifetime ISA

The Lifetime ISA sits between the two. You can open one from 18 until you turn 40, pay in up to £4,000 a year until you are 50, and the government adds a 25% bonus, at most £1,000 a year. The £4,000 counts toward the £20,000 ISA allowance (HMRC, Lifetime ISA 2026).

Withdrawals are free of charge for a first home costing up to £450,000, from age 60 or if you are terminally ill. Any other withdrawal costs a 25% charge. For first homes above the £450,000 cap, the charge falls to 20% from October 29, 2026 (HMRC, Lifetime ISA 2026).

For new savers, the Lifetime ISA is on its way out. The Budget of October 28, 2026 announced a First Time Buyer ISA with a 25% government bonus from April 2029, which will replace the Lifetime ISA for new savers; existing holders can keep paying in under the current rules. As of November 3, 2026, this is an announcement, not law (Budget 2026, overview of tax legislation 2026).

Inheritance tax and other changes

Pensions used to be the best way to pass on wealth, because unused funds were largely outside the estate for inheritance tax; the IFS found the treatment of pensions at death very generous (Adam et al. 2022). For deaths from April 6, 2027, most unused pension funds and death benefits count toward the estate. The change is law in the Finance Act 2026, with death-in-service benefits excluded (HMRC, inheritance tax on pensions 2026). Anyone who planned to spend the ISA first and leave the pension untouched for heirs should review that order.

Salary sacrifice is also changing. From April 6, 2029, National Insurance will be due on pension contributions made through salary sacrifice above £2,000 a year, while income tax relief stays as it is (HMRC, salary sacrifice 2025). The Budget of October 28, 2026 also announced a Pension Tax Allowance from April 2027, meant to keep pensioners whose income is no higher than the full new State Pension out of income tax (Budget 2026 2026); it is not yet law. The lesson is that pension rules move. An ISA has fewer rules that can change, which is part of its value.

A practical order

1. Take the full employer match in a workplace pension. It is extra pay that neither wrapper can replace.
2. Keep an emergency reserve in cash, in a cash ISA if your interest would exceed the personal savings allowance.
3. Higher-rate taxpayers: pension contributions first, as far as they earn relief at 40%.
4. Income between £100,000 and £125,140: each £2 of income above £100,000 removes £1 of personal allowance (HMRC, income tax rates 2026), so the marginal rate in this range is 60%, and pension contributions that bring income back to £100,000 get relief at that rate.
5. Basic-rate taxpayers: a mix. The pension wins on tax if you expect to stay a basic-rate taxpayer in retirement, the ISA keeps money available. Under 40 and saving for a first home, the Lifetime ISA's bonus beats both.
6. Money you may need before 57: the stocks and shares ISA.

Common mistakes

• Comparing the SIPP's gross value with the ISA's net value: compare what each leaves after tax, starting from the same take-home pay.
• Forgetting to claim higher-rate relief: relief at source only adds 20%; the rest has to be claimed through Self Assessment.
• Taking a large taxable withdrawal in one year: it can push income into the 40% band.
• Triggering the money purchase annual allowance by accident while still planning to pay in more than £10,000 a year.
• Using a Lifetime ISA for money you may need before 60: the 25% charge takes back more than the bonus.

This article explains general UK tax rules for 2026/27 as of November 3, 2026, and is not financial or tax advice. Tax treatment depends on your circumstances and may change; Scottish taxpayers pay different income tax rates.

This article explains a method; it is not investment or tax advice. Worked examples are simplified and rounded. All terms in the glossary.