ISA vs SIPP: Which Should You Fill First in 2026?

The ISA vs SIPP question comes up in almost every GIP portfolio review where the client has spare monthly capital to invest. It sounds like a simple either/or question. It is not.

The right answer depends on your current marginal tax rate, how many years you have before retirement, whether you might need the capital before 57, your estate planning position, and since the October 2024 Autumn Budget whether the pension IHT changes from April 2027 affect the calculus for your specific situation.

This post gives you the decision framework. The numbers are real. The analysis reflects what I work through with GIP clients in practice.

Alpesh Patel OBE is a hedge fund manager, Bloomberg TV alumnus, Financial Times author, and former Visiting Fellow at Corpus Christi College, Oxford. The ISA vs SIPP question features in virtually every GIP client review for investors aged 40–60 with active ISA and SIPP portfolios.

ISA and SIPP: The Core Tax Difference

The fundamental difference between an ISA and a SIPP is when the tax relief occurs:

• SIPP: tax relief is received on contributions going in. A higher rate (40%) taxpayer investing £10,000 receives £4,000 in tax relief, meaning the effective cost of £10,000 in the SIPP is £6,000. Growth is tax-free. Withdrawals in retirement are taxed as income (except the 25% tax-free cash). No access before age 57 (rising from 55 to 57 in 2028).
  

• Stocks and Shares ISA: no tax relief on contributions going in. Growth is tax-free. Withdrawals are completely tax-free. Full access at any time with no age restriction.

The SIPP’s advantage is the upfront tax relief. The ISA’s advantage is unrestricted flexibility and completely tax-free withdrawal. Both grow in the same environment — no income tax, no capital gains tax on investments inside the wrapper. The question is whether the upfront tax relief on the SIPP contribution more than compensates for the income tax payable on retirement withdrawals.

The Higher Rate Taxpayer Case: SIPP First, Every Time

For a higher rate (40%) taxpayer with 10 or more years to retirement and no near-term liquidity requirement, the SIPP is almost always the better first choice. The maths is compelling:

You invest £10,000 of after-tax income. In a Stocks and Shares ISA, £10,000 goes in and grows. In a SIPP, a 40% taxpayer can claim back £4,000 in tax relief, meaning the £10,000 contribution cost them £6,000 net. At 13% growth over 15 years, that £10,000 grows to approximately £54,000. The 25% tax-free cash on withdrawal (£13,500) is tax-free. The remainder (£40,500) is taxed at the rate applicable in retirement which for many GIP clients will be 20% basic rate, not 40%. So the effective tax on withdrawal is approximately 15% of the total — against 40% relief received on contribution. The net advantage is significant.

The Basic Rate Taxpayer Case: It Depends

For a basic rate (20%) taxpayer, the SIPP still provides 20% relief on the way in. But if withdrawal in retirement is also at 20% (which it often is for those who remain basic rate taxpayers throughout retirement), the tax advantage is more modest. The calculation is:

• SIPP: 20% relief on the way in, 25% of the pot tax-free on the way out, 20% on the remainder. Net advantage exists but is more modest.

  
  

• ISA: no relief on the way in, 100% tax-free on the way out, and full flexibility before retirement. For basic rate taxpayers who may want access to the capital before 57, or who expect their retirement income to push them into higher rate territory, the ISA’s flexibility has material value.

The April 2027 Factor: How the IHT Change Affects the Decision

Until April 2027, unused SIPP pots sit completely outside the estate for IHT — making the SIPP the most tax-efficient vehicle for intergenerational wealth transfer. From April 2027, unused pension pots will be included in the estate and subject to IHT at 40% above the nil-rate band.

This change partially erodes the SIPP’s estate planning advantage. For investors who were deliberately leaving the SIPP undrawn as an IHT-efficient vehicle — drawing from ISAs and other assets first — the calculus changes from April 2027. The SIPP still has the contribution tax relief advantage and the in-retirement tax-free cash advantage, but the IHT-free death benefit advantage is significantly reduced.

Post-April 2027, the ISA becomes relatively more attractive for estate planning purposes — ISA assets remain outside the taxable pension pot and continue to be IHT-free (to the extent they fall within the nil-rate band and other exemptions). For investors with large pension pots who are already above the IHT threshold, using ISA contributions to build a more flexible, estate-planning-efficient pool of assets makes increasing sense.

The GIP Decision Framework: Which to Fill First

• Higher rate taxpayer, 10+ years to retirement, no near-term liquidity need: Max SIPP first (up to the annual allowance), then ISA with surplus.
  

• Basic rate taxpayer, 10+ years to retirement: SIPP for the tax relief benefit (20% + 25% tax-free cash in retirement); ISA for flexibility and potential estate planning. Split based on liquidity needs.
  

• Any taxpayer, less than 7 years to retirement: The SIPP’s access restriction becomes more relevant. ISA contributions provide flexible capital that can supplement retirement income without being locked up until 57.
  

• Large estate, IHT exposure: From April 2027, ISA becomes more attractive relative to additional SIPP contributions for investors already at or near the IHT threshold. Prioritise ISA for estate efficiency post-2027.
  

• The optimal answer for most GIP investors: use both. Max the annual allowance on the vehicle with the highest marginal advantage for your tax position, then fill the other. The ISA’s £20,000 annual allowance and the SIPP’s £60,000 annual allowance (2026/27) are not in competition — they are complementary.

Frequently Asked Questions: ISA vs SIPP

Should I put money in an ISA or a pension?

For most higher rate taxpayers with 10+ years to retirement: pension (SIPP) first. The 40% relief on the way in, combined with the 25% tax-free cash on the way out and likely lower rate taxation on the remainder, creates a significant mathematical advantage. For basic rate taxpayers or those needing capital flexibility, an ISA-first or split approach may be better. Both wrappers should ideally be used — they serve different roles.

What is the ISA allowance for 2026/27?

The ISA annual allowance remains £20,000 for 2026/27. This can be split across a Stocks and Shares ISA, a Cash ISA, an Innovative Finance ISA, and a Lifetime ISA (up to £4,000 in the LISA). The £20,000 limit applies to the total across all ISA types in a single tax year.

What is the SIPP annual allowance for 2026/27?

The pension annual allowance is £60,000 for 2026/27 (including both employer and personal contributions), subject to the money purchase annual allowance (MPAA) of £10,000 if you have already flexibly accessed pension benefits. Higher earners may be subject to the tapered annual allowance, which reduces the annual allowance for those with adjusted income above £260,000.

Can I withdraw from a SIPP before 57?

Generally no. The minimum pension access age is currently 55 (rising to 57 in 2028 for most SIPP holders). Accessing a SIPP before the minimum access age normally triggers an unauthorised payment charge of up to 55% plus a scheme sanction charge. Exceptions apply only in cases of serious ill health or terminal illness. For investors who may need capital before 57, an ISA provides unrestricted access that the SIPP does not.

Does it matter which order I withdraw from ISA and SIPP in retirement?

Yes, significantly. Drawing from the ISA first in retirement — which is tax-free — and leaving the SIPP to continue compounding is typically the optimal strategy for investors with both wrappers. The SIPP continues growing in a tax-free environment while the ISA provides tax-free income. This strategy also, under current rules, preserves the SIPP’s IHT advantage until April 2027 - after which the optimal sequencing may need to be revisited in light of the new pension IHT rules.

For a personalised review of your ISA and SIPP position, your current contribution strategy, and how to apply the GIP framework across both wrappers, book a free portfolio review here.

Sources & Further Reading

HMRC — ISA rules, allowances, and tax treatment. gov.uk/individual-savings-accounts

HMRC — Pension tax relief, annual allowance, and lifetime allowance rules for SIPPs. gov.uk/tax-on-your-private-pension

HM Treasury — Autumn Budget 2024: pension IHT changes from April 2027. gov.uk/government/publications/autumn-budget-2024

Financial Times — ISA vs pension comparison, tax efficiency, and estate planning. ft.com/personal-finance

MoneyHelper — ISA and pension comparison: tax relief, access, and retirement income planning. moneyhelper.org.uk/en/savings/types-of-savings/isas

Which? — ISA vs pension: which is better for saving for retirement? which.co.uk/money/pensions-and-retirement

Disclaimer: This article is for educational purposes only. Tax rules are subject to change. The April 2027 pension IHT changes are based on the October 2024 Autumn Budget and the final legislative detail may differ. This does not constitute personal financial or tax guidance. Individual circumstances vary significantly — the framework above is a general guide, not a personal recommendation.

Alpesh Patel OBE

www.campaignforamillion.com